Update dhcp agent cache for network:dhcp ports

Update dhcp agent cache for network:dhcp ports

When a network with a dhcp_enabled subnet is scheduled on a dhcp
agent, dhcp driver creates the network:dhcp port for the subnet.
However, the port info is not updated in dhcp agents internal cache.
Subsequently if the user deletes the network:dhcp port, the port is
properly deleted on the server side (i.e., in the database) and when
the port_delete_end notification is sent to the dhcp agent, it simply
ignores it as the port entry would be missing in the cache. This patch
fixes this issue by updating the dhcp agents cache when dhcp driver
creates the network:dhcp port for the subnets.

Closes-Bug: #1478426
Change-Id: I69f5834dd964a4320c606c4e0aa2cdba70416943