Add capability to provide an alternate path for sudo

This addresses a need to provide an alternate path for sudo. With this approach, only one os can override the sudo path at a time. A more general approach is probably not warranted since this is really an unusual situation. My attempt to further generalize was unsuccessful because the Exec.check_os method always uses sudo. Otherwise I could have registered hosts with os and alternate sudo path.

In a better world, it wouldn't be necessary to provide an alternate path for sudo.  :-(