[FIXED JENKINS-20227] Avoid submitting polling jobs if all suitable exec...

This concerns SCM polling with plugins that do require a workspace for polling.
The previous functionality was that if the the last used workspace is offline at the polling time, a new build is submitted, in order to create a workspace - for pollling. That is done inrespective of whether a suitable slave ( or executor ) is online at that moment

Added functionality that checks if any the desired slave(s) by label is online, if not NO_CHANGES is returned, and we will wait until next polling.
If the project is not restricted in terms of where it can run, any slave (executor) that is online is accepted, and the polling job is submitted