refactor: naming of garden name, identity and alias

[sven-petersen]

What would you like to be added:

Suggestion:

• rename config.Garden:Name to Identity: The cluster identity can be any kind of ID (potentially also a UUID) which might not be human readable.
• rename config.Garden:Alias to Name: An optional, user-provided name which can be used instead of the identity for e.g. targeting the cluster
• use the (new) name in logs/output as the identity can also be a UUID/any opaque ID which is hard to read/understand by the user. Fallback to the identity if no name is specified.

Why is this needed:

Better semantics.

@holgerkoser FYI, as discussed.