If n-api-meta is being run, remove from NOVA_ENABLED_APIS

If running n-api-meta as a separate service we shouldn't run it inside
of n-api. This patch is in support of Iddd44f7ee43b9287a788dea49eaa484316f8da04

Change-Id: I8a54cf13dc6083b78e89c9ea5413d9e4d8d4b37a
Related-Bug: #1270845
Merge "If n-api-meta is being run, remove from NOVA_ENABLED_APIS"
Merge branch 'master' of https://github.com/openstack-dev/devstack