Add attribute stores for glance-api.conf

Attribute name: stores
File name: /etc/glance/glance-api.conf
Section: glance_store

- known_stores is deprecated for stores option in Juno, update template
- Attribute for configuring stores does not exist, add it
- Spec test for stores attribute does not exist, update api-spec.rb
- Move default_store to [glance_store] section to meet definition

Change-Id: Ib3955e2efdff7f03c652d3438f43a98cbfe2c05e