Move to restart_on_change context manager when available

[David Ames]

Work will be done to make the existing charmhelpers.contrib.opensttack.utils.pausable_restart_on_change [0] both a decorator and a context manager which then can be generalized in charms.openstsack [1]

Work on LP Bug#1915842 [2] will use pausable_restart_on_change as a decorator.

TRIAGE

When pausable_restart_on_change is a context manager switch to that method.

[0] https://github.com/juju/charm-helpers/blob/master/charmhelpers/contrib/openstack/utils.py#L1750
[1] https://github.com/openstack/charms.openstack/blob/master/charms_openstack/charm/core.py#L821
[2] https://bugs.launchpad.net/charm-placement/+bug/1915842