Need to allow constraints for selection of just a few build scenarios

[jimklimov]

While the original goal of Dynamatrix is to check as many different combinations as possible by agent capabilities, there may be some scenarios that are okay to check only once wherever is able to do so, or in a few combos and not exhaustively, to keep overall build times in check:

• spellcheck can be done just once (currently that stage is coded largely outside the dynamatrix idea)
• when systems provide many compiler versions, some faster scenarios (e.g. main branch iterations?) might use just the newest one (and/or maybe the oldest one) based on the version numbers declared here or there.
  • make a way to compare C/C++ standard revisions (e.g. numbered by year) to also check the newest variant available on that used compiler (we already know for gcc and clang who supports what ranges well)

Implementation idea: use the remaining build combo factors (OS family/distro...) as a map key, so as the value for each we can assign the newest ${COMPILER}VER version available on that system - note we need a way to cater for auto-substituted values here, e.g. COMPILER above can expand to separately CLANGVER and GCCVER.

[jimklimov]

Updated dynamatrix to handle a customized commonLabelExpr so a different set of agents can be inspected for (also customizable, e.g. reduced to not iterate {COMPILER}VER for docs builds) dynamatrixAxesLabels they serve, and other data.

Along with excludedNodelabels and requiredNodelabels filters, this addresses much of the concern above, by carefully crafting the "slowBuild filter" definition.

It still does not let constrain the build combos by spec desired from issue description though.