JCLOUDS-707: Improve SSH key pair creation in DigitalOcean

https://issues.apache.org/jira/browse/JCLOUDS-707

The SSH key pair creation  in DigitalOcean has been improved to avoid having multiple public keys with the same fingerprint:

* Create the SSH key pairs before creating the nodes.
* Only auto-generate key pairs if the user has not configured a public key.
* If the user has provided a public key, try to find one with the same fingerprint and reuse it.
* To make the migration to v2 easier, now the fingerprint of the public key is used for the name of the key pair.
[minor] "public key cannot be null"?