Adding tests for sqlalchemy class

Increasing UT coverage by adding tests on the SQLAlchemyDBApiTestCase
suite (most of them, related to allocation and reservation)
Adding test for event_get_all_sorted_by_filters and
host_allocation_get_all_by_values (including both sorting types)

Change-Id: Ibbf1ada814b38bdb8b12249f9e9f0b854feea4fe
Closes-Bug: #1259559
Merge "Adding tests for sqlalchemy class"