Simpler path for developers to tweak settings

[James Polley]

Currently, devtest.sh writes a $TRIPLEO_ROOT/tripleorc file which contains some of the standard settings which developers might want to tweak - but *only* after a devtest.sh run completes. Getting this completed run is increasingly tricky - and usually requires that developers tweak some of the settings in the tripleorc file *before* the run.

Because of this, we tell new developers (in https://wiki.openstack.org/wiki/TripleO#Notes_for_new_developers and elsewhere) to first create a ~/.devtestrc which has some of the settings, then source that, then proceed with devtest.sh

It would perhaps be simpler if devtest_variables.sh wrote the tripleorc as soon as it ran, at the top of the devtest.sh run instead of at the end. We could then tell devs to start by sourcing that file, then editing the generated tripleorc to make their changes. If we were to do this, we should probably include some of the other frequently-required settings (as listed in https://wiki.openstack.org/wiki/TripleO#Helpful_infrastructure) in the file, commented out, with a pointer to docs on how to figure out a good setting.