add a way to 'kill' stuck agents

[vchepkov]

Please consider adding a capability to kill a stuck puppet agent - those with the catalog applier process never dying.

[ripienaar]

For this we can do 2 things:

• Add an action to kill the applying process if the lock file exist and has a pid in it
• Add a new plan to compliment the find_stuck_agents one that can also kill them