Implements blueprint use-common-timeutils
1. Edit openstack-common.conf and import glance/openstack/common/timeutils.py
2. Replace datetime.utcnow with timeutils.utcnow
3. Replace utils.isotime with timeutils.isotime
4. Replace utils.parse_isotime with timeutils.parse_isotime
5. Replace utils.normalize_time with timeutils.normalize_time
6. Remove datetime related functions and datetime related unittests
Change-Id: I014548848f738b00c670be3d372b16f47f6dee96