Add interface_attach compute feature flag

Dynamic attachment of network interfaces is not currently a supported feature
in the baremetal case.  This adds feature flag to skip related API tests.

Change-Id: Ia09e84d7acfcd7291b3c9b58bc163603d6a0fb34
Merge "Add interface_attach compute feature flag"
Merge branch 'master' of https://github.com/openstack/tempest