Add osapi_compute_link_prefix to nova.conf

When a load balancer is used to terminate SSL, it generally
strips the https:// from the HTTP_HOST header and changes it
to http://. This is used by webob.Request as the request_url
that is then used by default when constructing link collections
in the OpenStack Compute API. Setting the osapi_compute_link_prefix
configuration option allows us to override the URI returned to
match the original compute endpoint URI.