[digital_ocean] Don't die when the id parameter is missing

It's okay for this to happen now, because we might move on to the name parameter
if unique_name is enabled.
Add basic domain handling to digital_ocean cloud action
[digital_ocean] revert original module and add one new module for each command
[digital_ocean] Remove command usage from digital_ocean_ssh examples

I seem to have forgotten to do this in my last commit.
Rename digital_ocean_ssh to digital_ocean_sshkey per https://github.com/ansible/ansible/pull/4315#issuecomment-30286556
Merge pull request #4315 from mgregson/add-digital-ocean-domains

Add digital ocean domains