Update tools/build-image.sh with some useful options
Update tools/build-image.sh with some useful options

Mainly I always forget to set the nameservers; the corporate firewall
disallows outbound DNS queries so this breaks image build after the
values are written into /etc/resolv.conf

Update with a few other options & clarifications too.  All should be
self-explanatory.

Change-Id: I359f10f2d80cbc8997580870cf3de88903c5f507

openstack-infra/project-config,Tesora/tesora-project-config,Tesora/tesora-project-config,openstack-infra/project-config