Fix set_ssh_keys.py

[tuladhar]

Please review following changes:

• Fix get_permission() call to return permission string of given argument path. Also make use of st_mode attribute which removes need to import stats module to make code shorter and simpler.
• Fix permission checking if statement which always returns False as get_permission() call returns permission string and we're checking against octal permission value.

[vpetersson]

LGTM. @ntassev, can you take a look?

[jjlopez]

It was merged on https://github.com/cloudsigma/pycloudsigma/pull/17.