Stack identity reads tenant from stack not context

This refactors the way stack identity reads tenant_id information so
that it uses the information already in the stack, instead of getting it
from the context every time.

This allows for unscoped list of stacks to show correct project
information (the project that each stack belongs to), instead of the
current one in the context.

Co-Authored-By: Richard Lee <rblee88@gmail.com>
Closes-Bug: 1294385
Change-Id: I976e7f7dea4432b97a2b97382bf05eb3bd1ca565
Merge "Stack identity reads tenant from stack not context"