Add option to return exit code from rt:admin calls

Add an rt:admin/3 function that accepts a list of options as the third
parameter.  Currently the only valid option is return_exit_code. The
rtdev, rtssh, and rt_cs_dev harnesses have been updated to support
this option. If return_to_exit is specified the return from a
?HARNESS:admin call is a pair with the exit code as the first member
and the command output as the second member. Finally the
basic_command_line test has been changed to use return_for_exit to
verify the changes.
Merge branch 'feature/riak-admin-exit-code-option'