Check for existence of screen before start/stop

Running devstack in the devstack gate environment doesn't
create a screen for the running services. This causes the start,
and stop screen functions in redstack to fail. Fix this by only
performing the start and stop functions if a screen exists.

Partial-bug: 1386337
Change-Id: I2a87e7409575d1000286a7a3742d29a9cc877a8e
Merge "Check for existence of screen before start/stop"