Update power_type for trusty

The combination of fence-agents and cobbler shipped in Ubuntu trusty
uses different names for selecting power types than were used with
Ubuntu precise. Update cobbler.yaml to reflect new names.

Change-Id: Ib02a6c0d5f9017e6cf8ef3c09f3780d0a0a8be9e
Closes-Bug: #1325030
Merge pull request #89 from CiscoSystems/bugs/1325030

Update power_type for trusty