switch-to-containers: do not fail when stopping the nfs-ganesha service

switch-to-containers: do not fail when stopping the nfs-ganesha service

If we're working with a jewel cluster then this service will not exist.

This is mainly a problem with CI testing because our tests are setup to
work with both jewel and luminous, meaning that eventhough we want to
test jewel we still have a nfs-ganesha host in the test causing these
tasks to run.

Signed-off-by: Andrew Schoen <aschoen@redhat.com>
(cherry picked from commit b613321c210155f390d4ddb7dcda8dc685a6e9ea)
Signed-off-by: Sébastien Han <seb@redhat.com>