Add missing tests for db.block_device_mapping_* methods

There was no tests in test_db_api for block_device_mapping_* methods.
Add tests to ensure that all works. Also these tests allow us to refactor
work with session in these methods.

Fix bug in is_ephemeral we should return True or False not instance of
_sre.SRE_Match or None.

blueprint db-api-tests

Change-Id: I3cac17e7d2c8d0b2608fb9a78ea29210f025222b
Merge "Add missing tests for db.block_device_mapping_* methods"