Move enabled_apis option into nova.service

The enabled_apis option is only used in nova-api and nova-all but
there's no more obvious place to put it than nova.service. At least
this location is consistent, since we quite have a number of other
options related to the APIs in nova.service.

blueprint: scope-config-opts
Change-Id: I86e200fa5ad91ac0b7ab6837cc61e5927b2ebeb7
Merge "Move enabled_apis option into nova.service"