lib/quantum: refactor quantum plugins and third party

As quantum plugin support is coming like floodlight, nvp and nec,
it's worth while to refactor quantum plugin logic so that each plugin can
be modified/enhanced intervening with other quantum plugin.
And new plugin support can be added easily (hopefully) without modifying
core logic.

Change-Id: Ic5ab5b993272fdd3b4e779823323777a845ee681