PE-1178: --security-group option handling improvements

This patch improves the handling of the``` --security-group``` option of the ```puppet node_aws create``` command.
Previously it was only possible to specify the security groups by their names, newly it is possible to specify them  by both IDs and names interchangeably. Internally they are always converted to IDs and as such passed to the ```Puppet::CloudPack.create_server``` method in the ```:security_group_ids``` option. (Previously the names of the groups were passed in the ```:security_group``` option to that method.)
The patch also updates the relevant tests and adds new ones, and while at it the tests for the entire ```create``` action of the ```node_aws``` puppet face are cleaned up and modernized.