[docker] deploy.sh: Use tag for stable branch

[docker] deploy.sh: Use tag for stable branch

Outside OPNFV Jenkins (i.e. when manually cloning the OPNFV Fuel
repo and starting a deploy), the Docker tag used to default to
'latest' unless the user specifically set it to 'gambia'.
Rely on 'defaultbranch' setting in .gitreview to determine the
appropiate Docker tag.

Change-Id: I7e6b0706597d84d7cd5dc077499da78031aa61af
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
(cherry picked from commit 4ad7cf5046bbfb90d11f6c172475d75fad257149)