One call in localloop requires the output of the command, so modify
exec_sudo to buffer up output and return it. This is modelled on the
same thing in package-installs-v2 which seems to work. Rather than
return a subprocess exception, return a dib exception which everything
should have imported anyway.
The overall reason for this is to make our external calls more
consistent for mocking in unit testing.
Change-Id: I10d23b873dee9f775daef2a4c8be5671d02c386e
a1a549548a