Refactor and reorganize riak_test_escript module

Refactor the riak_test_escript module to improve code clarity and
organization.  This includes renaming functions to be more reflective
of their purpose and breaking large functions into smaller
functions. Also add the ability to specify a comma-separated list of
tests with the -t option.
Fix issue with lager startup
Merge branch 'riak-test-escript-refactor' into feature/decouple-cluster-setup