Add missing tests for db.fixed_ip_*(). functions.

Added new tests for fixed_ip*() functions.
Function fixed_ip_create() should return FixedIp instance rather than
FixedIp.address, because it is used in a tests later.
Fix calls of fixed_ip_create() respectively.

blueprint test-db-api

Change-Id: Id5e9d4486605bde9d9fd1e631d868603a7a7c604
Merge "Add missing tests for db.fixed_ip_*(). functions."