Add deep-merge to pallet.utils
Add :release to package-source doc string
Fix zero arity nodes-in-group
Add explain-phase

Adds pallet.repl/explain-phase which can be used to see the action plan
resulting from the execution of a phase from a server-spec (or group-spec).
Ensure :package-source install updates packages

When using the :package-source install method, ensure that the package-update
occurs before the package install if it is required.
Correct log text in executors
Update to pallet-fsmop 0.2.6-SNAPSHOT
Fix group-name crate function
Enable passing of group-name in explain-plan