Remove '--versions', run_on_cluster, build_command, sudo_args and cluster_run

* Once 'versions' is out, we don't need get_branches to verify its value.
* Once we don't need to set variables for subprocess invocations, we
  don't need 'env', and we don't need a build_command helper that turns
  it into a command line, or shell_map.
* We also don't need shell_map which converts a mapping to a string
  representation of environment variables.
* run_on_cluster (not to be confused with cluster_run) just delegates to
  cluster_monitor and logs if the command fails on some hosts. But
  cluster_monitor already does that.
* cluster_run was never used.
* dsh is no longer used.
* sudo_args is throwaway.
* Don't time trivial operations.
* Get rid of unused 'uncommon' stuff.

Change-Id: I956385c24271597f8781b2a88bd31fe6abfa932c