Stop returning atoms from execute/revert methods

It is not needed to return the atom that was executed from the
futures result() method, since we can just as easily set an
attribute on the future and reference it from there when using
it later. This is also required for a process based executor since
it is not typically possible to send back a raw task object (and
is not desireable to require this); even if it was possible the
task would be pickled and unpickled multiple times so when this
happens it can not be guaranteed to even be the same object (in
fact it is not).

Part of blueprint process-executor

Change-Id: I4a05ea5dcdef97218312e3a88ed4a1dfdf1b1edf
Merge "Stop returning atoms from execute/revert methods"