Introduce EntityOwnerSelectionStrategy

Introduce EntityOwnerSelectionStrategy

Currently the EntityOwnershipService does not do any load
balancing, in that it allows the first candidate that registers
to become an owner. There is a need to do that so that applications
which choose to do some *work* based on if it owns an entity can
scale better.

This patch introduces the concept of an EntityOwnerSelectionStrategy
with the intent to provide custom strategies later to choose an owner.

Since custom strategies require intimate knowledge of how the
EntityOwnershipShard chooses a leader at this time I do not think
a strategy can be passed to the EntityOwnershipService via API. The
intent therefor is to choose a strategy based on configuration
wherein a custom strategy can be chosen for each entity type. If
the Strategy needs any custom configuration then it can have configuration
files of it's own

Change-Id: Ia53b8edb59fb1d06a426d9d9a95c07ef4ae65cd1
Signed-off-by: Moiz Raja <moraja@cisco.com>