Add support to run nova-api-metadata as separate binary

nova-api-metadata is commonly run as a separate binary in production and
devstack already has support to do this. Just wire up devstack-gate to
support this mode.

Config patch: Id7169e9ec72a934c2c2116f6e6156419b51b104f

Change-Id: Iddd44f7ee43b9287a788dea49eaa484316f8da04
Related-Bug: #1270845
Merge "Add support to run nova-api-metadata as separate binary"