Reconfig failure if not buildslave meets the criteria

[Georges Racinet]

In case a set of criteria (build-requires) is met by no buildslave, reconfiguration fails because that's not allowed by buildbot:

2014-11-18 23:29:05+0000 [-] builder 'deb-producer-amd64': at least one slavename is required

This is a useless trap, and further, it makes the creation of new builds less fluid than it could.

In that case, my priority was indeed to guarantee first that this build does not run on a wrong system, then bring up a buildslave safely.

This behaviour should be easy to bypass : do not issue a builder if the list of matching slaves is empty, output a warning, and that's it.

[Georges Racinet (gracinet)]

Actually that was only if the non meeted criteria were in conjunction with the newly introduced 'postgresql not-used' requirement option.