(maint) Don't overwrite mast_opts hash

This test was resetting overwriting the master_opts hash being sent into
with_puppet_running_on, accidentally relying on the broken Beaker 1.7
behavior which would delete the cmdline args from the passed hash.  This
fix should allow this test to run with Beaker 1.8+
Merge pull request #2556 from jpartlow/maint/master/fix-cmdline-overrides-environment-test

(maint) Don't overwrite mast_opts hash