Add a tunable to enable/disable tenant tracking

According to https://docs.google.com/document/d/1BrLIyxemIlr4h1XM2z0A-SAM6GZuQRo4OQXrVEWaad0/edit ns_server plans to enable/disable DBaaS mode to enforce limits by:

curl -s -u test_usr1:asdasd http://localhost:9000/internalSettings -d "enforceLimits=true"

They need to make that setting visible to memcached through memcached.json. The file don't use CamelCase for variables, and by itself isn't very describing so we'll use "enforce_tenant_limits_enabled" in the API between ns_server and memcached.

   "enforce_tenant_limits_enabled" : <bool>