(QENG-969) Add modify_tk_config DSL method.

Comes with some "private" helper methods for reading tk config strings and
merging or replacing the SUT config file using Beaker::Options::OptionsHash

Signed-off-by: Wayne <wayne@puppetlabs.com>
(QENG-969) Update `with_puppet_running_on` spec test.

Test that `with_puppet_running_on` actually uses `modify_tk_conf` when the
circumstances are right.

Signed-off-by: Wayne <wayne@puppetlabs.com>