Issue: simultaneous usage of 'shell' and 'run_as_root' options led
to error.
Cause: it's actually a concealed TypeError: in 'shell'
mode the command argument is assumed to be a string, elsewhere
a list/tuple.
Fix: the command editing implied by 'run_as_root' is performed
with care taken of above tacit type assumption.
Change-Id: Iaba33e6fda67793ae2874ba278c990271ee5e47f
Closes-Bug: #1382873