Wait until MinIO is up before proceeding

[Didayolo]

Fix bugs mentionned in #771

codabench-createbuckets-1   | Waiting for minio to startup...
codabench-createbuckets-1   | + echo 'Waiting for minio to startup...'
codabench-createbuckets-1   | + sleep 0.1
codabench-createbuckets-1   | + sleep 3
codabench-createbuckets-1   | + /usr/bin/mc config host add minio_docker http://minio:9000 testkey testsecret
codabench-createbuckets-1   | mc: <ERROR> Unable to initialize new alias from the provided credentials. Server not initialized, please try again.
codabench-createbuckets-1   | + /usr/bin/mc mb minio_docker/public
codabench-flower-1          | Traceback (most recent call last):
codabench-createbuckets-1   | Bucket created successfully `minio_docker/public`.
codabench-createbuckets-1   | + /usr/bin/mc mb minio_docker/private
codabench-createbuckets-1   | Bucket created successfully `minio_docker/private`.
codabench-createbuckets-1   | + /usr/bin/mc policy set download minio_docker/public
codabench-createbuckets-1   | mc: Please use 'mc anonymous'