Simplify config

[kulmann]

With this PR, the OCIS_TRACING_* config variables become the global config for all services. If you want to set individual tracing config for each service, set OCIS_TRACING_ENABLED=false (which is the default) and configure tracing for all services individually.

Tracing documentation updated as well, to reflect both different usages (global config and per-service config).

Implementation for owncloud/product#92

[kulmann]

I have this in my oCIS config file:

"Tracing": {
          "Enabled": true,
          "Endpoint": "localhost:6831",
          "Collector": "http://localhost:14268/api/traces"
  },

which works...

[kulmann]

Accounts service doesn't have a tracing config so far.

[kulmann]

@refs would be nice to get your opinion on this...

also: needs a rebase

[kulmann]

rebased + added jaeger endpoint and collector as default values

[butonic]

but proxy is still not sending spans, tracked in https://github.com/owncloud/ocis-proxy/issues/61

[refs]

We might want to look at this struct. cfg.Reva.Reva seems redundant. Issue for another time :)

[kulmann]

ocis-store command will be aligned with other commands after #409 is merged. The simplified tracing config for ocis-store has to be done later. Tracked here: #445

[kulmann]

rebased to current master.