radosbench should not depend on ceph-common to be installed on head-node

cbt checks the rados version to determine the syntax of the object size for rados bench. It does so locally on the head node (by running rados -v) which fails if the rados binary (from ceph-common on Red Hat systems) is not installed.
Instead cbt should ask the first client node for the version, if it assumes all clients to have the same rados version installed.

Start at cbt's rados version check and its local `rados -v` invocation. Trace how the first client node is selected and verify that version detection runs there instead of on the head node. Done means rados bench syntax detection works when ceph-common is absent from the head node.

Written by the indexing model from the issue text.