Make bucket/prefix part of host definition

[klauspost]

Add bucket/prefix part of the --host parameter instead of separate. Make it mandatory.

Never clear anything at or below what is specified.

Buckets must be created before starting.

[suykerbuyk]

I hacked an approximation of the functionality here: https://github.com/minio/warp/compare/master...suykerbuyk:master

[harshavardhana]

This is not needed anymore..