settings: enable "ibft" plugin by default together with "ifcfg-rh"

settings: enable "ibft" plugin by default together with "ifcfg-rh"

Originally, ibft settings were handled by "ifcfg-rh" plugin. Later, we added
a separate "ibft" plugin and moved the functionality there.

The problem was that users quite possibly had a configuration like
  [main]
  plugins=ifcfg-rh
in their "NetworkManager.conf". That meant, after upgrade users would
no longer have ibft support.

We fixed that by installing "/etc/NetworkManager/conf.d/10-ibft-plugin.conf"
which was read after the main file and contained:
  [main]
  plugins+=ibft

We no longer want to install configuration snippets with our core packages to
/etc. Avoid the regression by changing the meaning of "ifcfg-rh". By enabling
"ifcfg-rh" you now implicitly enable "ibft" plugin as well. This can be
turned off via "no-ibft". And you can continue to enable "ibft" plugin
alone.