Caching of meta user does not seem to work

[gwossum]

Caching of meta user does not seem to work. Even when a user should be cached, the auth service will still get the user from the meta API.

The following lines of test code should work without an gock request for Alice, but fail because the meta API caching is not working: https://github.com/influxdata/kapacitor/blob/3407c75c78a6c0ac6a5ca32949d12d9a0e60b57e/integrations/metaauth_test.go#L182-L187