Fix handle_create of NetDHCPAgent for updating

When NetDHCPAgent is updated with new network_id which is already
associated with the dhcp_agent_id, UPDATE_FAILED occurs in Heat. Fix
handle_create to check associating information to avoid this situation.

Closes-bug: #1272258
Change-Id: Ic827295e773d893d9af6d7cf1c687aee69e98298
Merge "Fix handle_create of NetDHCPAgent for updating"