Add salt.utils.parse_docstring() to parse dependencies out of docstrings

Fixes #3727

Currently only parses out dependencies, for use in required_module_list().  Can be added to later as needed.

Should now work for the previous 'Required python modules: ' method, and the RST ':depends:' method.  Does not parse ':optdepends:' as of yet, as that's not currently needed.

Also use parse_docstring in required_module_list (fixes #3727)