core: Updated Transaction _enter_ method

This patch updates the Transaction class method
_enter_ by adding a return of 'self'. This allows
using the Transaction object in a context manager
in a following way:

 from otopi import transaction

 with transaction.Transaction() as mytransaction:
     mytransaction.<class methods or objects>

Change-Id: I83825d6f92e029ba6acec7ab0387a18370a55ee5
Signed-off-by: Alex Lourie <alourie@redhat.com>