api functions using labels

[buschNT]

Some API functions are only possible using the cluster name. Sometimes this requires multiple interactions from the beiboot-api to implement basic user based functionality (which is implemented based on labels) since it is always required to validate the cluster name first. Furthermore, actions like "delete all clusters from user 'test'" are not possible. Thus I suggest to add the following functions:

api.read_one(labels)
api.delete(labels)

Maybe this observation holds true for other functionality such as connect, heartbeat. However, I haven't encountered this problem there yet.