Fix problem of deleting dhcp port

when network is deleted, the dhcp port in that network will be deleted
automaticly, so '_cleanup_ports' should not clean dhcp port. I already
have a patch to fix the problem of deleting dhcp port.

Change-Id: I92f90a3dad6d76954d466e4b30ab7c46434ba7df
Closes-bug: #1279683
Merge "Fix problem of deleting dhcp port"