Add "gpgkeypath" option to remotes

For Project Atomic, we already have RPM signatures which use files in
`/etc/pki/rpm-gpg`.  It's convenient to simply bind the OSTree remote
configuration to those file paths, rather than having duplicate key
data.

This does mean that we need to parse the files for verification, so we
end up importing them into the verifier's temporary keyring, which is
a bit ugly, but it's what other projects do.

Closes: https://github.com/ostreedev/ostree/issues/573

Closes: #575
Approved by: giuseppe

man/ostree.repo-config.xml

@@ -199,7 +199,8 @@ Boston, MA 02111-1307, USA.
   <refsect1>
     <title>Per-remote GPG keyrings and verification</title>
     <para>
-      OSTree supports a per-remote GPG keyring.  For more information see
+      OSTree supports a per-remote GPG keyring, as well as a
+      <literal>gpgkeypath</literal> option.  For more information see
       <citerefentry><refentrytitle>ostree</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
       in the section <literal>GPG verification</literal>.
     </para>

man/ostree.xml

@@ -433,8 +433,12 @@ Boston, MA 02111-1307, USA.
           in this directory.
         </para>
         <para>
-	    In addition to the system repository, OSTree supports a
-	    per-remote
+	  In addition to the system repository, OSTree supports two
+	  other paths.  First, there is a
+	  <literal>gpgkeypath</literal> option for remotes, which must
+	  point to the filename of an ASCII-armored key.
+	</para>
+	<para>Second, there is support for a per-remote
 	    <filename><replaceable>remotename</replaceable>.trustedkeys.gpg</filename>
 	    file stored in the toplevel of the repository (alongside
 	    <filename>objects/</filename> and such). This is