Installing openvswitch on an x86/64 snapshot image pulls in a bunch of
dependencies, good for a total size of 3648406 byte. Disabling
libunbound reduces that with 559941 byte, for a total of 3088465 byte.
This is quite a big reduction for a small tradeoff: without libunbound,
hostnames can not be used to specify OpenFlow managers or controllers.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>