slurmrestd - restrict ability of requests to use different authentication plugins.

Enforce the result of calls to rest_auth_context_apply() to ensure that
any calls to slurmrestd can not potentially use a different authentication
plugin. Avoid defaulting to auth/munge unless in local authentication
context.

Clear authentication context in parse_http() instead of calling
rest_auth_context_apply() which will now fail.

Bug 9206.