(MCO-466) Add search of $libdir/mcollective/agent/$agent/$action to impl...

...emented by

The commit d9d4354 implemented a default search path for scripts
supplied as parameters to #implemented_by. However the path did not
correspond with the path described in the feature ticket
http://projects.puppetlabs.com/issues/13064

This commit adds the previously described search path with the aim of
deprecating the incorrect search path in the future.
The full path to the script is logged in line 42, just before the command is run. 

Example: 
 DEBUG -- : actionrunner.rb:42:in `run' Running test_agent#run command: /vagrant/marionette-collective/plugins/mcollective/agent/test_agent/test.sh