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
Allow Resources to select paths from attributes

Previously when get_attr included a path to select from the attribute, the
selection was done by the get_attr intrinsic function. Starting with the
Juno version of HOT, move control of this to the resource plugin, so that
individual resources (particularly ResourceGroup) can handle it themselves.

Related-Bug: #1341048
Change-Id: Ic3dc94a99a19e28a4f775198ff6e960e2efb8256
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
Prevent excessive validation for maxPersonality limit

We have PERSONALITY property on OS::Nova::Server that defaults to {},
but code that triggers check for limits compares value of property
to None -> this property always get validated for limits (if limit is
set).

Change-Id: I552a03cd9f4ab570aa059349ab361bb50c9c0d1e
Closes-bug: #1333283
Fix rollback failure when updating stack

When building oldstack in rollback, self.env doesn't match self.t, then
the validating will fail, but oldstack is useless in rollback, so we don't
need to build it in rollback.

Change-Id: Iab17824d108d565126c1dd0338fefc9a5ee4f47b
Closes-Bug: #1308883
Merge "Allow Resources to select paths from attributes"
Merge "Fix rollback failure when updating stack"
Merge "Exceptions ResourceInError, ResourceUnknownStatus"
Merge "Added reference to the Nova Flavor documentation."