remove unnecessary cleanup in nova startup
remove unnecessary cleanup in nova startup

nova's instances directory may be a shared directory (ie nfs), in
these cases, we do not want to call nova_cleanup at startup since it
deletes everything under $NOVA_INSTANCES_PATH.

The nova_cleanup routine will still be called by the clean.sh script
which is fine since we're presumably cleaning up the whole openstack
cluster at that point.

Change-Id: Ieb4e5d0508d4ed4c5349c497554c5da2993c9cb0
Closes-Bug: #1649389

nawawi/openstack,group-policy/devstack,noironetworks/devstack,openstack-dev/devstack,dtroyer/devstack,noironetworks/devstack,nawawi/openstack,dtroyer/devstack,group-policy/devstack,openstack-dev/devstack