Add option to provide a custom tls.Config to allow control over the tls connection

[jwatson-gcu]

This PR proposes two things:

1. Adding a service.ServiceOption to provide a tls.Config to NewNimbleGroupService() which is in turn used by resty.Client

   The current behavior hard codes the tls connection to skip certificate verification, which is something that is not allowed in our security environment.

	restyClient.SetTLSClientConfig(&tls.Config{
		InsecureSkipVerify: true,
	})

2. When using a Tenant User, certificates are fully validated.

Signed-off-by: Joseph Watson jwatson@genisyscu.org

[jwatson-gcu]

Example usage:

	nimbleClient, err := service.NewNimbleGroupService(
		service.WithHost(config.Host),
		service.WithUser(config.User),
		service.WithPassword(config.Password),
		service.WithTLSConfig(tlsConfig),
	)