Make P2P API public (no longer experimental)

Currently the API that allows P2P operations (e.g. pulling an ostree ref
from a LAN or USB source) is hidden behind the configure flag
--enable-experimental-api. This commit makes the API public and makes
that flag essentially a no-op (leaving it in place in case we want to
use it again in the future). The P2P API has been tested over the last
several months and proven to work.

This means that since we're no longer using the "experimental" feature
flag, P2P builds of Flatpak will fail when using versions of OSTree from
this commit onwards, until Flatpak is patched in the near future. If you
want to build Flatpak < 0.11.8 with P2P enabled and link against OSTree
2018.6, you'll have to patch Flatpak.  However, since Flatpak won't yet
have a hard dependency on OSTree 2018.6, it needs a new way to determine
if the P2P API in OSTree is available, so this commit adds a "p2p"
feature flag. This way the feature set is more semantically correct than
if we had continued to use the "experimental" feature flag.

In addition to making the P2P API public, this commit makes the P2P unit
tests run by default, removes the f27-experimental CI instance that's no
longer needed, changes a few man pages to reflect the changes, and
updates the bash completion script to accept the new commands and
options.

Closes: #1596
Approved by: cgwalters

man/ostree-find-remotes.xml

@@ -58,13 +58,11 @@ Boston, MA 02111-1307, USA.
     <refsect1>
         <title>Description</title>
 
-        <!-- FIXME: Reword this when no longer in experimental -->
         <para>
             OSTree has the ability do pulls not just from configured remote
             servers but also from peer computers on the LAN and from mounted
-            filesystems such as USB drives. Currently this requires compiling
-            OSTree with experimental API enabled, and it requires the use of
-            collection IDs and GPG verification.
+            filesystems such as USB drives. This functionality requires the use
+            of collection IDs and GPG verification.
         </para>
         <para>
             The <command>find-remotes</command> command searches for remotes

man/ostree-summary.xml

@@ -84,12 +84,10 @@ Boston, MA 02111-1307, USA.
                         fields to the summary.
                     </para>
 
-                <!-- FIXME: Uncomment this when collection ID support becomes non-experimental.
                 <para>If the repository has a collection ID configured, the
                   <filename>ostree-metadata</filename> branch for that collection ID
                   will also be updated with a new commit containing the given metadata,
                   which will be signed if the summary file is signed.</para>
-                -->
 
                 </listitem>
             </varlistentry>

man/ostree.xml

@@ -283,14 +283,13 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
-            <!-- FIXME: Uncomment this when it's no longer experimental
             <varlistentry>
                 <term><citerefentry><refentrytitle>ostree-create-usb</refentrytitle><manvolnum>1</manvolnum></citerefentry></term>
 
                 <listitem><para>
                     &nbsp;Put the given refs on an external drive for P2P distribution.
                 </para></listitem>
-            </varlistentry>-->
+            </varlistentry>
 
             <varlistentry>
                 <term><citerefentry><refentrytitle>ostree-diff</refentrytitle><manvolnum>1</manvolnum></citerefentry></term>
@@ -301,14 +300,13 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
-            <!-- FIXME: Uncomment this when it's no longer experimental
             <varlistentry>
                 <term><citerefentry><refentrytitle>ostree-find-remotes</refentrytitle><manvolnum>1</manvolnum></citerefentry></term>
 
                 <listitem><para>
                     &nbsp;Find remotes to serve the given refs.
                 </para></listitem>
-            </varlistentry>-->
+            </varlistentry>
 
             <varlistentry>
                 <term><citerefentry><refentrytitle>ostree-fsck</refentrytitle><manvolnum>1</manvolnum></citerefentry></term>