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
(cherry picked from commit 1b66e11b5d8c0b3de0610ca02c3e10b6f64ae375)
Merge "Create DHCP port for IPv6 subnet" into stable/juno