Add concept of "staged" deployment

Add API to write a deployment state to `/run/ostree/staged-deployment`,
along with a systemd service which runs at shutdown time.

This is a big change to the ostree model for hosts,
but it closes a longstanding set of bugs; many, many people have
hit the "losing changes in /etc" problem.  It also avoids
the other problem of racing with programs that modify `/etc`
such as LVM backups:
https://bugzilla.redhat.com/show_bug.cgi?id=1365297

We need this in particular to go to a full-on model for
automatically updated host systems where (like a dual-partition model)
everything is fully prepared and the reboot can be taken
asynchronously.

Closes: https://github.com/ostreedev/ostree/issues/545

Closes: #1503
Approved by: jlebon

tests/installed/destructive/staged-deploy.yml

@@ -0,0 +1,24 @@
+# Test the deploy --stage functionality
+
+- name: Write staged-deploy commit
+  shell: |
+    ostree --repo=/ostree/repo commit --parent="${commit}" -b staged-deploy --tree=ref="${commit}" --no-bindings
+    ostree admin deploy --stage --karg-proc-cmdline --karg=ostreetest=yes staged-deploy
+  environment:
+    commit: "{{ rpmostree_status['deployments'][0]['checksum'] }}"
+- include_tasks: ../tasks/reboot.yml
+- name: Check that deploy-staged service worked
+  shell: |
+    # Assert that the previous boot had a journal entry for it
+    journalctl -b "-1" -u ostree-finalize-staged.service | grep -q -e 'Transaction complete'
+    # And that we have the new kernel argument
+    grep -q -e 'ostreetest=yes' /proc/cmdline
+- name: Rollback
+  shell: rpm-ostree rollback
+- include_tasks: ../tasks/reboot.yml
+- shell: |
+    ostree refs --delete staged-deploy
+    rpm-ostree cleanup -rp
+# And now we shouldn't have the kernel commandline entry
+- name: Check we do not have new kernel cmdline entry
+  shell: grep -qv -e 'ostreetest=yes' /proc/cmdline