Scheduler hints are not really documented very well at all except
for being mentioned per scheduler filter in the admin configuration
guide, nor are they documented within relation to flavor extra
specs which are both used for impacting scheduling decisions and
are choices that a deployer has to make based on how they configure
their cloud.
This change adds a document about scheduler hints and how they are
similar to and different from flavor extra specs, including end
user discoverability and interoperability, and thoughts on which
should be used if writing a custom scheduler filter.
The TODO in the API guide is also resolved by linking to this
document.
Change-Id: Ib1f35baacf59efafb9e4bccfcc4f0025d99ad5b2
579e0cfd57