dhcp-agent: equalize port create_low/update/delete priority

Low port delete priority may lead to duplicate entries in network
cache if IPs are reused frequently.
Also can't find a strict reason why it should be of lower priority.

Change-Id: I55f858d50e636eb9091570b256380330b9ce9cb3
Related-bug: #1862315
Related-bug: #1828423

neutron/api/rpc/agentnotifiers/dhcp_rpc_agent_api.py

@@ -44,8 +44,8 @@ PRIORITY_SUBNET_DELETE = 4
 # other dhcp agents.
 PRIORITY_PORT_CREATE_HIGH = 5
 PRIORITY_PORT_CREATE_LOW = 6
-PRIORITY_PORT_UPDATE = 7
-PRIORITY_PORT_DELETE = 8
+PRIORITY_PORT_UPDATE = 6
+PRIORITY_PORT_DELETE = 6
 
 METHOD_PRIORITY_MAP = {
     'network_create_end': PRIORITY_NETWORK_CREATE,