Create DHCP port for IPv6 subnet

There is a bug in dhcp agent code that when first port is created
in an IPv6 subnet, DHCP port is not automatically created.

This fix resolves this problem by removing the IP version check
in configure_dhcp_for_network method.

Change-Id: If3f405d367a7099d9f33d72d11ffcb7a393abe23
Closes-Bug: #1367500
Merge "Create DHCP port for IPv6 subnet"