Convert the DateTime into ISO8601 format for Ceilometer

The fields of DateTime type should be converted to the time in
ISO8601 format, when they are put in the usage information to be
reported to Ceilometer.

Change-Id: I67e9e3609291d0d835156dfd9cfda88deae119c1
closes-bug: #1372791
Merge "Convert the DateTime into ISO8601 format for Ceilometer"