lib/sysroot: Add OSTREE_EX_STAGE_DEPLOYMENTS environment variable

I feel like I'm drowning in a pile of experimental-but-almost-stable
features...

Anyways, since we made the feature opt-in in rpm-ostree in
https://github.com/projectatomic/rpm-ostree/pull/1352
let's mirror that a bit here with an environment variable so people
can play with it more easily.

The tests needed some tweaks; specifically we need to reload the
status fact after making changes.  I'm still a bit uncertain
about the Ansible-as-tests.

But we add an upgrade test that uses the new environment variable.

Closes: #1583
Approved by: jlebon

tests/installed/tasks/install-git.yml

@@ -8,7 +8,8 @@
   synchronize: src=build/x86_64/ dest=/root/x86_64/ archive=yes
 - name: Install RPMs
   shell: rpm-ostree override replace /root/x86_64/*.rpm
-- include_tasks: ../tasks/reboot.yml
+- import_tasks: ../tasks/reboot.yml
+- import_tasks: ../tasks/query-host.yml
 - command: ostree --version
   register: ostree_new_version
 - set_fact:

tests/installed/tasks/query-host.yml

@@ -1,4 +1,5 @@
-- command: rpm-ostree status --json
+- name: Load status json
+  command: rpm-ostree status --json
   changed_when: False
   register: rpmostree_status_json
 - set_fact: