as we move toward deprecating classic drivers,
we need a way to specify which particular combination
of interfaces of a hw type to assign to nodes being enrolled
and run test against.
This patch adds a number of IRONIC_DEFAULT_<>_INTERFACE
variables to devstack plugin (all empty by default)
and if set, sets their value as "[DEFAULT]default_<>_interface"
config option.
Change-Id: I9fc22125ddd3c70ff37185d025df1dfa3217a782
aa5fd8afdc