Add simple escript to modify advanced.config.

In the future we should fix the tests to actually use a non-test
backend configuration properly, but for now we can apply our
customization directly to the advanced.config file, even against
riak_ee which will ship with settings already in advanced.config.
Copy advanced.config overlay logic from cuttlefish
Messed up the shebang but didn't put it back.
ETOOMUCHNESTING
Merge pull request #188 from basho/bugfix/sdc/merge-advanced-config

Merge advanced.config change instead of overwrite

Reviewed-by: seancribbs