This adds a helper method that modules can call to execute a command via
subproces.  It takes two arguments: the command to run and
keyword options that control how the process is executed.  Supported
options are: fail_on_rc_non_zero, close_fds, and executable.
fail_on_rc_non_zero will call fail_json if the command fails.  If
args is a list, the command will be run with shell=False; otherwise, if
a string, it will be run with shell=True.  Otherwise, run_command() returns
the returncode, stdout, and stderr.