Make linux.utils.execute log error on return codes

Previously, the execute method in neutron logs everything as debug which hides
a lot of extremely fatal errors like unable to apply security group rules!
This patch changes this code so that we log all non 0 returns as error.

Change-Id: I7328e62269212ccd9c7950ff064a3e337de56918
Closes-bug: 1323832
Related-bug: 1322945
Merge "Make linux.utils.execute log error on return codes"