Add broker-specific control for insecure communications

[pmorie]

Currently, we hard-code the InsecureSkipVerify option on the TLS config for the broker client's HTTP transport to allow communicating with brokers who do not have a root-signed certificate. We probably need to retain this capability, but it should be broker-specific. Something like:

type BrokerSpec struct {
  Insecure *bool
}

[pmorie]

Oops, this is a dupe of #606