This change add an os-vif lib that declares two new variables
OS_VIF_OVS_OVSDB_INTERFACE and OS_VIF_OVS_ISOLATE_VIF
The former is introduced to workaround bug #1929446 which cause the nova
and neutron agents to periodically block waiting for ovs to respond.
OS_VIF_OVS_ISOLATE_VIF is added to address bug #1734320 when using
ml2/ovs vif isolation should always be used to prevent cross tenant
traffic during a live migration. This makes devstack more closely
mirror reality by enabling it when ml2/ovs is used and disabling it
otherwise.
Related-Bug: #1734320
Related-Bug: #1929446
Related-Bug: #1912310
Change-Id: I88254c6e22b52585506ee4907c1c03b8d4f2dac7