Use strict=True instead of `is_valid_boolstr`

Oslo's `bool_from_string` learned the `strict` keyword which allows
callers to detect invalid boolean values, so we can use that instead
of having a new Nova-specific function.

Change-Id: I61bfa4029897c7304bd54d6cdae9f9a9bc4c1f78
Remove unnecessary parens in test_volumes

Return statements were using unnecessary parens, this removes them.

Change-Id: Iee63ffa826d323a89493e16c1b9592bae74e350f