Synchronize GlobalRuby#evaluate (TORQUE-1078)

When deploying multiple applications we were letting multiple threads
interleave their GlobalRuby#evaluate calls when resolving ERB in
-knob.yml files which could sometimes lead to
java.lang.ArrayIndexOutOfBoundsException. The root cause of the
exception seems to be a thread-safety issue in either the ERB library
or JRuby string concatenation, but this should workaround things for
now.
Make at_jobs_spec a bit more resilient to slow CI servers