Add unique constraint to AgentBuild.

Added unique constraint 'uniq_agent_builds0hypervisor0os0architecture0deleted'
('hypervisor', 'os', 'architecture', 'deleted') to AgentBuild model and
migrate sripts. Added new exception `AgentBuildExists`.
Updated agent_build_create(). Tests updated respectively.

blueprint db-enforce-unique-keys

Change-Id: I6c41378470230a48773da8082b17699b09785051
Merge "Add unique constraint to AgentBuild."