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
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"