Small PEP8 fixes
Add salt.utils.parse_docstring()

Currently only parses out dependencies, for use in
required_module_list().  Can be added to later as needed
Use parse_docstring in required_module_list (fixes #3727)

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
Merge pull request #4480 from basepi/docstrparse

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