If we get passed a non-string stack name, e.g a map or list, we
fail with a DB error associated with looking up the existing stack.
So instead force all stack lookups to use string identifiers, and
make the name validation for new stacks robust to fail gracefully
when there is an invalid (non string) argument passed.
Change-Id: I052dc4a715773895d070e1e9f26183c6a1cf3d7f
Closes-Bug: #1533065