There are always some questions about how quotas work
in Nova, particularly how they are checked against
the configuration vs the 'default' quota class in the
database, and global vs project-specific quotas.
This change starts a document on quotas a high-level.
It does not get into the lower-level details of how
quotas work or are designed in Nova, or the issues
that can arise with the existing quota system or future
plans.
Change-Id: Iabe3d40f44c59f14fa89ceddc556497d2885845c
fdf10d63bc