Allow null settings in ES when using SetClusterSetting

[leosunmo]

Sorry I've been sitting on this one for a while, got busy.

This adds the ability to "null" settings when using SetClusterSetting().
fixes #68

The function signature is now

SetClusterSetting(setting string, value *string) (*string, *string, error)

and if you pass a nil *string as the new value it will reset that setting in Elasticsearch to its default value.

It also returns a *string as the existingValue and newValue. This allows you to save the original state of a potential null/unset setting and use it in a second call to SetClusterSetting() to reset the original state after the operation has finished.

oldSetting, _, err := v.SetClusterSetting("indices.recovery.max_bytes_per_sec", "1000mb")
  if err != nil {
    log.Printf("failed to set recovery max bytes cluster setting")
    return
}
defer func() {
  v.SetClusterSetting("indices.recovery.max_bytes_per_sec", oldSetting)
}()

In this example the indices.recovery.max_bytes_per_sec is probably unset (default) and should be reset to its null state when the shard migration is complete.

Other random stuff:

• Added debug message if there's an issue in the test server function buildTestServer, I had an extra space in the body and it took me way too long to figure it out.
• Added a small check to the integration test script file.
• Added stringToPointer(v string) *string helper function.
• New go.mod entry for github.com/spf13/pflag, not sure how this one wasn't already there as it's required by github.com/spf13/cobra.

Passes unit tests and integration tests.

[nickcanz]

Thanks a lot @leosunmo, this looks great!