Fix metadata proxy start problem for v6-v4 network

Fix metadata proxy start problem for v6-v4 network

Commit 1b66e11b5d8c0b3de0610ca02c3e10b6f64ae375 introduces
a new problem that metadata proxy will not be started when
an isolated network contains ipv6 subnet with dhcp enabled
and a ipv4 subnet.

This fix separates the checks to enable dhcp and enable metadata
so metadata proxy can be started if there is ipv4 subnet exists
in the network with metadata enabled.

Change-Id: If26806083db225e9ab30d75fa6c728ce99a7cafe
Closes-Bug: 1392564
(cherry picked from commit 4c128c351589ef9f2fc1e084c655a97bdbbda7ce)