MB-100: Edit performance tests

- Enter threadsNumber along with other bucket
parameters, while creating a bucket through rest
- threadsNumber being the disk read-write
concurrency

Change-Id: I8666da00acc08ad86690accc6e14d06ff1b9c17d
Reviewed-on: http://review.couchbase.org/27184
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>