DBaaS: Remove hardcoded default version for creates.
DBaaS: Remove hardcoded default version for creates.

The API itself will default to a sane version for the
specified database engine. By using a hard-coded default
of 11, attempts to create MySQL or Redis clusters without
explicitly specifying the version will fail. Eg:

$ doctl databases create --engine mysql doctl-mysql
Error: POST https://api.digitalocean.com/v2/databases: 422 (request "a11fa359-5778-42a5-8474-d776c5c097fd") invalid cluster engine version

digitalocean/doctl,digitalocean/doctl