Introduce DEVSTACK_GATE_UNSTACK env variable

When this variable is set to "1" environment created by devstack will be
removed by calling ./unstack.sh.  This is beneficial for installing
dependencies while not being interested in running openstack services.

Change-Id: I47cd842edc44858e6f487a425de0ab7a68ba4cf8
Related-Bug: #1360658
Merge "Introduce DEVSTACK_GATE_UNSTACK env variable"