CentOS 7 issues w/ pcs / CIB shadow

[wyardley]

[Via @actatux; creating an issue to track separately from #395]
The acceptance test with docker/centos-7 fails because of an issue with pcs and CIB_shadow.

A regression has been introduced in el 7.4, see https://bugzilla.redhat.com/show_bug.cgi?id=1497736

There is a fix ongoing at RH. I guess a fix should land to CentOS in the next months or so.

However, we will have to rework the cs_shadow/cs_commit mechanism for pcs provider because it is not supported upstream. See the comment in the patch proposal of RHBZ#1497736.

# Pass the CIB defined in the CIB_shadow environment variable which was
# previously (i.e. in the old architecture) passed because the whole
# environment was passed. This is only a workaround for users to accomodate
# to CIB_shadow not being passed anymore and it will be removed shortly. We
# cannot just pass CIB_shadow to the library, it would stop working when
# pcs splits into client-server as both a client and a server would have
# different environment (and possibly run on different machines).

I will propose a PR.

[btravouillon]

PR #410

[wyardley]

Closing via #410