add ns-metadata-proxy rootwrap filters to dhcp.filters

bug 1150628

now that dhcp-agent also uses ns-metadata-proxy, we need to make sure
those rootwrap filters are installed in scenarios where dhcp-agent is
installed by l3-agent is not.

Change-Id: I141b2291aceb93d650258ff1df1ef0122ce8da90
Merge "add ns-metadata-proxy rootwrap filters to dhcp.filters"