prevent irregular auth_prefix config in tempauth

Tempauth handles a authorization request by request path.
If a request's path start with auth_prefix, tempauth middleware
handles that request to authorization method.
Therefore, when configuring auth_prefix to '/', all requests
handle to authorization method.

This change enables tempauth to prevent invalid auth_prefix
config '/' and similar empty auth_prefix in initialization method.

Fixes bug #1096538.

Change-Id: I20b157e2a0809c17409fc65a8eff0858fe4aff29
Merge "prevent irregular auth_prefix config in tempauth"