Added support for Exclusion plugin

This plugin allows the specification of critical blocks for builders.
Once a set of builders is "wrapped" by a critical block start/critical
block end pair, any resources (string) specified in the exclusion build
wrapper will cause any jobs with those same resources to block and allow
only one of those builds to proceed at a time.

Change-Id: I0bf4ab003709917aa7fe6ad57b31200f466658da
Merge "Added support for Exclusion plugin"