enable use of environment variables in the command-line arguments read from test.config files of regression tests
In regression tests, if no data-dir defined on command-line, the tests will place the test results (blockchain database/logs) in a directory parallel to the bitshares_toolkit directory called "regression_tests". The first parameter on the command-line will be used as a sub-directory to store the results for each client in a given test.
Added brief introductory docs for how to create and run regression tests. Eliminated unnecessary "regression_tests" subdirectory from regression_tests_output directory hierarchy.