NetworkManager with simple-init has proven to be stable in OpenStack
infra, switch to it by default for CentOS and Fedora. For CentOS 8
and Fedora, add a check to make it the only option. Thus only CenOS 7
remains optionally using the legacy scripts; this is likely not used
anywhere (infra is really the primary user, where NetworkManager is
already used); we can likely remove this variable (and hence path) in
a future cleanup.
In the setup, remove rhel7 element which was never really tested.
Reorganise the fallthrough to call out the default paths as doing
nothing.
Change-Id: Ic996956da4b85f7d95179b8df9881d5f52c091af
cbe1a0fc6b