Make log level in linux.utils.execute configurable

Function linux.utils.execute always logs failures on error level.
Some callers use this function to check resource existance and
interpret failure as normal flow. Log level is made configurable
for such cases.

Change-Id: I8e4cd7b42234426f8fe58d353a64e898f02f8998
Closes-bug: 1339045
Merge "Make log level in linux.utils.execute configurable"