MB-100: POST to /logClientError actually needs admin creds

Because otherwise it's potential log spamming hole.

Change-Id: I4bade50a984351646c61b99d341225f5755da378
Reviewed-on: http://review.couchbase.org/27841
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>