[FIXED JENKINS-7291] Permit flyweight tasks to run on master even when it has zero configured executors.

Always adding Computer for master as a fallback

The original proposed fix for JENKINS-7291 creates a Computer object
transitively. This seems unwise as it violates the design of Computer
as stated in the javadoc, and for example we can end up creating two
Computers for the master.

I think a better fix is to create a Computer for the master all the
time, even if there's no executors configured. The discrimination in
Queue.makeBuildable would ensure that such phantom Computer is only used
as a last resort (statistically speaking).

I've also tweaked executors.jelly a bit. I simplified it somewhat
based on the idea that "if there's only one computer to show, the
context is likely making it obvious".

(I must be missing the intricacy in the current code.)

Originally developed in a branch at
2c5b57fcc1f39ed39057254e802f4183db5aa0dc then squashed for clarity.

as I was playing with it to understand what those logics mean.

aa432839.　

d8b29df3.　

f6a2f64b.　

5537213f.

updated online help for concurrent builds

added to online help about concurrent builds: a paragraph about using the hudson.slaves.WorkspaceList system property for configuring the separator

[FIXED JENKINS-17331] PerJobLocalRepositoryLocator should prefer the MavenModuleSetBuild.workspace for single-module builds.

[FIXED JENKINS-17084] Changelog should produce output even if some chang...

[FIXED JENKINS-17128] Unquote labels before processing