Respect inmutable resolv.conf in the image

In some instances, the content of the image may want to convey specific
resolv.conf settings. For instance, if unbound is installed, it's also
desirable to set resolv.conf to point to it.

Short circuit diskimage-builder's friendliness around resolv.conf files
by not dying in a fire if it finds the file to be immutable.

Change-Id: I88632fb79289681dcf95d32f39a0ad658ba39a60
Merge "Respect inmutable resolv.conf in the image"