Avoid traceback logs from simple tenant usage extension

Avoid generating traceback logs when invalid formatted datetime
parameters are passed to the simple tenant usage
extension. Exceptions have not been correctly handled by the
os-simple-tenant usage extension which results in tracebacks
being output to the log files. This patch correctly handles
exceptions as a result of badly formatted datetime strings.

Conflicts:
        nova/tests/api/openstack/compute/contrib/test_simple_tenant_usage.py

Change-Id: Iecd42239357b96450b6530c0486b27c68995c37f
Partial-Bug: 1300972
(cherry picked from commit f60c72fac04a97b321a7f2bd0865c1ae81250882)
Merge "Avoid traceback logs from simple tenant usage extension" into stable/icehouse