release start: stage branch doesn't need to exist
release start: stage branch doesn't need to exist

Fixing a bug. release start was requiring the staging branch to exist,
which doesn't need to be the case when the very first release is being
started.

Change-Id: f823771b4c
Story-Id: salsaflow/salsaflow#151

salsaflow/salsaflow,salsaflow/salsaflow