fix detection of required arguments

[dirk-thomas]

Before all arguments were considered required. With this change optional arguments are not counted when the signature length is checked,

This is required for ament/ament_tools#98 (which has already been merged and fails builds without this change).

[wjwwood]

The changes look ok to me, though the "pop-loop" is a bit strange (would a slice operation be more obvious?). Should the tests for this function be extended to cover this case?

https://github.com/osrf/osrf_pycommon/blob/master/tests/unit/test_cli_utils/test_verb_pattern.py#L9-L85

[dirk-thomas]

I used slicing instead and added a test for it. Please take another look.

[wjwwood]

lgtm