allow empty description attribute for os_security_group

The `os_security_group` module would fail if there was no `description:`
attribute:

    localhost | FAILED! => {
        "changed": false,
        "failed": true,
	"msg": "Error creating security group larstest: Invalid input for
	description. Reason: 'None' is not a valid string."
    }

This commit makes the default description `''` rather than `None`.

cloud/openstack/os_security_group.py

@@ -91,7 +91,7 @@ def _system_state_change(module, secgroup):
 def main():
     argument_spec = openstack_full_argument_spec(
         name=dict(required=True),
-        description=dict(default=None),
+        description=dict(default=''),
         state=dict(default='present', choices=['absent', 'present']),
     )