Boolean options: use strict checking

Boolean options (such as "--protected" for glance md-namespace-update)
should accept a limited amount of valid values, rather than assuming an
"invalid" value means "False".

The following values (no matter the case) will now be interpreted as
True: ‘t’,’true’, ‘on’, ‘y’, ‘yes’, or ‘1’.

The following values (no matter the case) will now be interpreted as
False: ‘f’, ‘false’, ‘off’, ‘n’, ‘no’, or ‘0’.

Change-Id: I0e7942045d883ac398bab4a7a85f2b4ac9b1ed8c
Closes-Bug: #1607317

doc/source/cli/property-keys.rst

@@ -27,3 +27,8 @@ in the Glance Administration Guide.
    For more information, refer to `Manage images
    <https://docs.openstack.org/glance/latest/admin/manage-images.html>`_
    in the Glance Administration Guide.
+
+.. note::
+
+   Boolean properties expect one of the following values: '0', '1', 'f',
+   'false', 'n', 'no', 'off', 'on', 't', 'true', 'y', 'yes' (case-insensitive).