lib/sysroot: Add OSTREE_EX_STAGE_DEPLOYMENTS environment variable

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