stestr migrate option is needed for easy migration from testrepository

[ssbarnea]

Issue description

The current documentation requires used to call a script that is included in the source, $STESTR_SOURCE_DIR/tools/testr_to_stestr.py which may be quite inconvenient.

Expected behavior and actual behavior

It would me much better if this would be executed via testr migrate instead as user would not have to know where is the source code installed.