Cleanup dhcp namespace upon dhcp setup.

Cleanup dhcp namespace upon dhcp setup.

In some cases when more than 1 DHCP agents were assigned
to a network and then they became dead, their DHCP ports
become reserved. Later, when those agents revive or start
again, they acquire reserved ports, but it's not guaranteed
that they get exactly same ports. In such case DHCP agent
may create interface in the namespaces despite that another
interface already exist. In such case there will be two
hosts with dhcp namespaces each containing duplicate ports,
e.g. one port will be present on two hosts. This breaks
DHCP.

Closes-Bug: #1509959
Change-Id: I34eb1ad5c44dd3528c9910462e26536186e7a4fb