engine : Introduction of Poller, CommandEntity and interfaces for caching

The poller interface eliminates all dependencies from the
AsyncTaskManager and the bll package

The CommandEntity has all required parameters to recreate
the command

The CommandEntityDAO is a place holder that will persist the
commands to the db

The method to remove command enities from the cache will
be introduced in later patches

Change-Id: I2172333255131652bb78623352fb805d16fb4a1b
Bug-Url: https://bugzilla.redhat.com/1083769
Signed-off-by: Ravi Nori <rnori@redhat.com>