move BackendPlugin interface into its own package

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
replace RunRunner with RunAction

This will ultimately be plugged into an ActionRunner that ExecuteAction
constructs, along with the other actions the 'actionrunner' package perform.

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>