Fix auth for CLI

[corylanou]

The auth for CLI was not setting credentials properly in the client package.

This PR does a couple things:

1. Refactored the ParseConfig logic in the cmd/influxd package to be more consistent. This was a side affect of trying to trace down where configs were parsed and was just to difficult to leave the current code.
2. Added a SetAuth to the client library to allow for resetting credentials.
3. Changed the client library to use basic authentication via headers instead of the uri. Effectively moving from http://golang.org/pkg/net/url/#UserPassword to http://golang.org/pkg/net/http/#Request.SetBasicAuth

Fixes #2239

[corylanou]

As a side note, there is no real way to write a test for this as both the cmd/influxd and cmd/influx are main packages, so I can't spin up a server in memory to test it. I'm open to other ideas if anyone has them.

[corylanou]

@otoolep previously, we had just NewConfig which wasn't specific enough between what we wanted for true server defaults, vs what we wanted to have to run in Demo/Test mode.

However, when we know we want a "Test" config (AKA Demo Mode), then we call that specifically. This was already happening behind the scenes before I made this change, but now you see it where it happens, which I like a lot more.

For reference, here is what we add when you ask for a test config:

func NewTestConfig() (*Config, error) {
  c := NewConfig()

  // By default, store broker and data files in current users home directory
  u, err := user.Current()
  if err != nil {
    return nil, fmt.Errorf("failed to determine current user for storage")
  }

  c.Broker.Enabled = true
  c.Broker.Dir = filepath.Join(u.HomeDir, ".influxdb/broker")

  c.Data.Enabled = true
  c.Data.Dir = filepath.Join(u.HomeDir, ".influxdb/data")

  c.Admin.Enabled = true
  c.Admin.Port = 8083

  c.Monitoring.Enabled = false
  c.Snapshot.Enabled = true

  return c, nil
}

[otoolep]

OK, thanks @corylanou, makes sense. This is a different change, but TestConfig isn't a great name IMHO, and is what threw me. It sounds like some sort of config used for our testing, not a default config that is used is no path is supplied. It's more like NewDefaultConfig.

[otoolep]

Otherwise change looks fine.

[corylanou]

Agree on the naming. it was between NewDemoConfig or NewTestConfig. Some day we may come up with a better name.

[toddboom]

lgtm

[otoolep]

tempfile() already removes the path, and cleaning up past runs is not possible, since the path changes each time.

[otoolep]

Looks good, just some minor feedback. Unfortunately I don't think we can clean up directories if a test panics, so that part of the change might be optimistic.