CLOUDSTACK-10294: PEP-8 fixes and enhancements to security_group.py (#…

…2432)

- We should return a boolean and not a String 'true' or 'false'. Although this output is never checked by the calling function(s).
- Do not use == False or == None as that is not according to the Python specs.
- Calling just print 'hello' is deprecated and won't work in newer Python versions. We should use the print() function.
- Remove unused and commented function.
- Use logging.warning() instead of logging.warn()
- Use subprocess.check_output() for execution. This is the Python way of executing commands.

Signed-off-by: Wido den Hollander <wido@widodh.nl>