Verify build nodes in YAML against config files

Verify build nodes in YAML against config files

Extend lf-jjb-verify anchor with boolean configuration variable
check-build-node-labels that guards a conditional build step.
If true, run script jjb-verify-build-nodes.sh to check build-node
labels in YAML files within the jjb subdirectory against labels
defined by config files in the jenkins cloud configuration
directory.  Disabled by default.  Projects should enable and
configure the job; e.g., for external build node labels.

Issue-ID: RELENG-2828
Change-Id: Iaf315b4a68c3488b5ead204a57361a8bcbca91d7
Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>