Add ability to exclude sets of specs

We recently needed the ability to exclude a directory of specs (in our
case, integration tests) from Buffet runs. Add an option to support
this.

Change-Id: I8ec2f4f6f9bcba50703058ec687c764a41cb4dc5
Reviewed-on: http://gerrit.causes.com/37317
Reviewed-by: Brian Mason <brian.mason@causes.com>
Tested-by: Shane da Silva <shane@causes.com>
Cut version 2.1.0

Change-Id: Ib0fb391b5393dbbc52da50ab7b3843bcfd555de4
Reviewed-on: http://gerrit.causes.com/37318
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>