Description of problem:
The ask here is to add a configuration option to tell the SSSD NSS responder to
not create the memory cache when the option is set.

We have seen issues with disk space in environments where sssd is restarted in
short intervals. It's clear that disabling the memory cache will have a
negative impact on NSS performance, but in some situations it might be helpful
to have such an option available.

Setting the "SSS_NSS_USE_MEMCACHE" env variable to "no" does not always help
because some services clear their environment before starting the actual
process. In this case the env variable is unset and the service is still using
the memory cache even though it has been told to not use it.

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