Make stable-interface-names its own element.

Virtual baremetal instances have their NICs show up as ens4
which doesn't play nicely as a default in devtest at
the moment.

Ideally we'd want it to be em1 like real baremetal
(which is why we put this in the baremetal element
to begin with). Turns out the baremetal element is
required in devtest so we can extract the ramdisk and kernel
so lets leave it alone for now and move this elsewhere
until we get a better solution for fake 'baremetal'
testing.

Closes-bug: #1298152
Change-Id: Ia71e1d32b93db0c4c844a6dc1ebcd04ab0c13f05
Merge "Make stable-interface-names its own element."