Masking expected failures

[ajdlinux]

As per discussion with mpe, longer term we need to look at ways for masking expected failures.
Some options:

• Know the state of the base branch, only report failures which have been introduced specifically by the new patch
• Allow maintainer to specify certain jobs/configs etc that will not be reported as failures

[ajdlinux]

@ruscur: "Maybe this is something that should be at the Jenkins level. If we can tag jobs as "dev" or "stable", we can compare the "dev" thing we just started building to the latest "stable" run and return a result from there accordingly.

This is something we need to address but it's very difficult and won't be for some time, I feel."