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
Handle errors from run_ofctl() when dumping flows

The function dump_flows_for_table() calls run_ofctl(). If this occurs during an OVS
restart, run_ofctl() will return None. dump_flows_for_table() needs to realize this
and not try to call splitlines() on a None object.

Change-Id: Ide26028656e634113f955dfc89569149d4d5b306
Closes-Bug: #1324659
Merge "Handle errors from run_ofctl() when dumping flows"
Merge "Make linux.utils.execute log error on return codes"