Makes security_group_rule_info compatible with new sdk version

- Changed get_security_group_rule to find_security_group_rule as
  get_security_group_rule throws an exception if the rule is not
  found
- Updated docs
- Updated tests
- Renamed ethertype to ether_type to match openstacksdk's attribute
  names and added the former as an alias to keep backward compat
- Renamed rule to id to match openstacksdk's attribute names and
  added the former as an alias to keep backward compat

Change-Id: Ieb99f875c990e11623c81e482013d0ecb8e61055