Add option to specify external net via ID

Created by: chess-knight

Fixes #572 (closed)

This commit should resolve these problems: OPENSTACK_EXTERNAL_NETWORK_ID was prefilled with external network name instead of ID what causes external net autodetection by CAPO - multiple ext nets cause error. Documentation says that 'external' can be name/id of external network which doesnt work - introduce external_id parameter. Autodetection of external network is improved - when multiple ext net exists, it can be specified via external/external_id