CBQE-975: Paramiko stdout.read() hangs-large output

Paramiko stdout.read() hangs while executing a large output command

upgrade using rpm -i/ dpkg -i generates a large stdout and testrunner hangs in execute_command_raw on stdout.read()

        for line in stdout.read().splitlines():
            output.append(line)
https://github.com/couchbase/testrunner/tree/master/lib/remote#L1155

I have no idea why channel approach is used in case of self.use_sudo, otherwise just self._ssh_client.exec_command(command)

Change-Id: I292ca923bd83ea5147a0f3b4e9b966efb0119259
Reviewed-on: http://review.couchbase.org/23907
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: buildbot <build@couchbase.com>