[#70844644] Don't set healthcheck URI for non-HTTP checks

In vCloud Director version 5.1 and 5.5, VMWare does not support a
healthcheck URI for SSL healthchecks[1]. In vCloud Director version 5.5,
a load balancer will refuse connections if a health check URL is set for
an SSL healthcheck.

TCP healthchecks obviously would not use a HTTP URI given they do not
operate at layer 7.

Also, raise an exception if a healthcheck URI is specified for a load
balancer healthcheck using any protocol other than HTTP.

This corresponds to similar changes in the 'vcloud-edge_gateway' gem
which is designed to replace 'vcloud-net-spinner'[2].

Note that vcloud-net-spinner doesn't currently support TCP load
balancers, so we only raise an exception for SSL healthchecks.

[1]: http://pubs.vmware.com/vcd-55/topic/com.vmware.vcloud.admin.doc_55/GUID-C12B3954-155F-48AF-9855-E0DE026752D0.html
[2]: alphagov/vcloud-edge_gateway@ecfba39