[#62380500] Basic schema for load balancer VSE config

includes tests for minimal *required* configuration.

Does not yet define the full input syntax.
[#62380500] LB schema correction

pools and virtual_servers can be empty, but must be specified
[#62380500] allow for multiple validating tests for LB schema
[#62380500] add schema tests for complete pool & virtual_server entries
Merge pull request #127 from alphagov/vse_lb_schema

VSE Load Balancer section schemas and tests.