Reqd. core_plugin for plugin agents & show cfg opts loaded.

Reqd. core_plugin for plugin agents & show cfg opts loaded.

For the linuxbridge, openvswitch, nec, and ryu plugin agents,
require the core_plugin configuration option, and throw an
exception, if not present. Note: cannot make this a required
option for all agents, as there are agent that include the
core config options, but do not require core_plugin.

In addition, log (as debug) the configuration options that
have been loaded at start-up for these four agents. This will
help with diagnosing start-up issues related to the
configuraiton settings (or lack of).

Tested that exception thrown, when missing core_plugin
value in config file(s) included, for these four agents.

bug 1059923
Change-Id: I813f0c2a3d0ec01be74a34e56ab085979197a16a