Avoid unnecessary explicit str() conversion around exceptions

Avoid unnecessary explicit str() conversion around exceptions

There are number of places like

except Exception as exc:
            LOG.error("Failed to get network: %s", str(exc))

where str() is not needed since %s substitution already does
the same conversion. Also LOG.error could be replaced with
LOG.exception, so argument exc won't be needed at all.

Closes-bug: #1398839

Change-Id: I73cc6e1ce55ade08e7706b99a5ab075f7059a4ef