Consider making cluster name optional in the API and generating a name

[elmiko]

Currently oshinko-s2i tooling does random cluster name generation for on-demand cluster creation, but this could just as easily be done in oshinko-rest if we make the cluster name optional instead of required on create.

Instead of a validation error, a missing name would cause a random name generation. The generated name would be returned in the result, so the caller would have full knowledge of the name chosen.

Any name passed would still be respected.

migrated from radanalyticsio/oshinko-rest#80

[rebeccaSimmonds19]

@tmckayus are you ok with this change for cli?