Add landscape-server machine constraints and updated integration to use haproxy reverseproxy endpoint

[d-kazantsev]

There are few changes in the terraform code in this PR:

1. Since no landscape-server machine constraints were added MAAS could select any machine from the available to use as a landscape-server machines. We need to distinguish them using tags. So machines constraints parameter was added to allow proper and predictable VMs selection.
2. It looks like there was no integration existing between legacy landscape-server rev 134 and haproxy via reverseproxy endpoint. I've modified endpount name in "landscape_server_haproxy" integrtaion resource.