Make it easier to skip multiple checks

Splitting an env var on space means escaping spaces.  It's more common to
separate env var values with colons for SKIP_CHECKS=foo:bar rather than
SKIP_CHECKS=foo\ bar.  SKIP_CHECKS now accepts ":", ",", or " ".

Change-Id: I034e1076557719507a9665b64af13b6f3ee752a7
Reviewed-on: https://gerrit.causes.com/19443
Reviewed-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Tested-by: Jeremy Dunck <jeremy.dunck@causes.com>