Add options for ZK ACLs.

[HoustonPutman]

Issue number of the reported bug or feature request: Resolves #143

Describe your changes
Adding a section in the SolrCloud.Spec.zkRef.connectionInfo for specifying a secret that contains a username and password for ZK ACLs (both admin and read only).

// ZookeeperSpec defines the internal zookeeper ensemble to run for solr
type ZookeeperACL struct {
	// The name of the Kubernetes Secret that stores the username and password for the ACL.
	// This secret must be in the same namespace as the solrCloud is running in .
	SecretRef string `json:"secret"`

	// The name of the key in the given secret that contains the ACL username
	UsernameKey string `json:"usernameKey"`

	// The name of the key in the given secret that contains the ACL password
	PasswordKey string `json:"passwordKey"`
}

These ZK ACL options are also available for the Prometheus Exporter.

Testing performed
Unit tests have been added for injecting the ACLs. It has also been tested locally.

[HoustonPutman]

Has unit tests, so the operator part is guaranteed to work. Also tested locally to make sure the solr and prometheus exporter params work correctly, as is expected.