Make hidden stack tag configurable

the current hardcoded value is different from the default one in Heat
and is mainly applicable to a Heat deployed with devstack-gate on CI.

To make this test pass against an OpenStack cloud in a general case
the tag that must be considered by the test to be hidden must be
configurable.

Change-Id: I51e03ee919536b5946271f2f9abc78e382dd00cb
Closes-Bug: #1757117
2 files changed