Remove duplicated agents in neutron database before doing db migration

Otherwise, we can hit an error like this during the migration:

sqlalchemy.exc.IntegrityError: (IntegrityError) could not create unique index
"uniq_agents0agent_type0host"
DETAIL:  Key (agent_type, host)=(Open vSwitch agent, d00-19-b9-e2-22-b5) is
duplicated.
 'ALTER TABLE agents ADD CONSTRAINT uniq_agents0agent_type0host UNIQUE
(agent_type, host)' {}

https://bugzilla.novell.com/show_bug.cgi?id=892503