Avoid performing extra query for fetching mac learning binding

Avoid performing extra query for fetching mac learning binding

Bug 1214879

Add a relationship performing eager load in the Port model, thus preventing
the 'extend' function from performing an extra query.
This patch also replaces assertTrue with assertEqual in unit tests as it
needs to evaluate whether the value of the mac_learning_enabled attribute
is equal to the boolean literal True, whereas assertTrue verifies that the
expression passed to it evaluates to True. This means that any value but
False will be enough for the test to pass, which is not correct.

Change-Id: I03345ef3a125fdfa1ec7f6c3363d76efc12ced82