support py3 when doing db_sync
support py3 when doing db_sync

when installing mistral on py3 environment
db_sync fails because python is used and not python3
should use the $PYTHON variable instead

see error
http://logs.openstack.org/99/491899/13/experimental/gate-vitrage-dsvm-datasources-py35-ubuntu-xenial/76dee18/logs/devstacklog.txt.gz#_2017-08-13_08_15_12_160

Change-Id: I61bd76b8d9ac48f13c6ca9b8be304c15cd8cf4d6

openstack/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral