Only setup dhcp interface if dhcp is not active on network

Only setup dhcp interface if dhcp is not active on network

When enabling (DhcpLocalProcess.enable()) dhcp for a network the agent
first sets dhcp interface, then checks if dhcp is curently active and
if it's true then the agent restarts dhcp.
Restart (DhcpBase.restart()) first disables dhcp and then enables it again
by calling DhcpLocalProcess.enable() recursively which in turn sets
dhcp interface again (it doesn't see the port created earlier as network
is not re-fetched from db). This leads to duplicate dhcp interface
for the network.

The fix is to only setup dhcp interface if dhcp is not active.

Closes-Bug: #1339968

Change-Id: I3a6d2fd5b18a97138ac5b699ecd4d4b30dbdbacb