Make use of unbound optional

In some 3rd party ci systems, the installation of unbound
cause node image creation to fail and/or devstack-gate to fail.

Since it is not strictly necessary, allow it to be disabled in
environments that do not support it.

Change-Id: I906ae3ccf946e208e17c7087f89641d645db7111
Partial-Bug: #1307702
Merge "Make use of unbound optional"