Add symlink test for resolv.conf restore
Add symlink test for resolv.conf restore

We replace the base resolv.conf with an "outside" copy so that
resolving works when we're in the chroot.

Installing resolvconf package modifies the in-chroot resolv.conf to a
symlink (to /var/run) which it wants maintained in the final image.
We have the existing "immutable" check for a created resolv.conf file,
but no eqivalent for a symlink.

This adds a check to see if the resolv.conf is a symlink and leave it
alone if it is, assuming it has been re-created in the chroot.

I have tested this with ubuntu-minimal+resolvconf with
dhcp-all-interfaces and the system seems to work with resolvconf
working correctly.

Change-Id: Idd5a26e9d55979bd951577d5b098ed4bfba91ad3

switch-ch/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder