Merge PR #1704 into master

Merge PR #1704 into master

* refs/pull/1704/head:
	teuthology/suite/test: test nested subsets
	teuthology: add option to disable nested subsets
	teuthology/suite: create nested matrix subsets
	teuthology/suite: patch builtin open method
	teuthology/test: use correct exception type
	teuthology/suite/test: make sure patchers are cleaned up on exception
	teuthology/suite/test: clarify variable name

Reviewed-by: Zack Cerza <zmc@redhat.com>