Do not support toggling key_manglers in cache layer

Due to needing to pass the PKI token (complete id) to the backend,
the cache layer needs to ensure that the cache-key is always
shortened down to a reasonable length. Disabling the key_mangler
should only have ever been done in limited debug configurations.
Real deployments would be unable to reliably run without the
key mangler (due to cache-key length on most systems being
exceeded by the Token calls).

This change of ID being passed around is for supporting
non-persistent tokens.

DocImpact: configuration.rst updated to reflect the change in
           configuration options for keystone cache layer.
bp: non-persistent-tokens

Change-Id: Ia4eb3df7ccffa58ee867120f698e24d926f0ec9e
Merge "Do not support toggling key_manglers in cache layer"