Added reference to the Nova Flavor documentation.

Updated contrib documentation with reference to the nova flavour README file.
Fixed the reference to the Docker documentation.

Change-Id: I23e352b8ec44424611066c0f64051ad3586b8c91
Exceptions ResourceInError, ResourceUnknownStatus

Resources which go into an unknown or error state currently
raise an Error exception. There are cases where Error is too
generic to make decisions on, so 2 new exceptions have been
created:
* ResourceInError: raised when a resource goes to a status which
  is known to represent an error for that resource
* ResourceUnknownStatus: raised when a resource goes to a status
  which heat is not aware of. If ResourceUnknownStatus is ever raised
  then this is a heat bug, since this is a status which heat is not
  aware of.

Any resource which go into an in-progress state has been updated to
raise ResourceInError and ResourceUnknownStatus where appropriate.

Change-Id: Ied83b030d30b8c26d9f4a1e454c71cd30715b0ba
Merge "Exceptions ResourceInError, ResourceUnknownStatus"
Merge "Added reference to the Nova Flavor documentation."