Add missing schema for virtual_server persistence

There was no schema defined for virtual_server persistence method,
which resulted in https://github.com/alphagov/vcloud-edge_gateway/issues/36
being raised.

This adds the missing schema, and tests to confirm it is working.

It reflects that COOKIE method is available via http, SSL_SESSION_ID via https, and
nothing via tcp.
re-add README section on persistence
Add http persistence section to our integ tests

Include the newly enabled persistence section in our
integration tests, as we don't use it but the wider community
does.
Add test cases surrounding tcp service_profile

As we are using individual schema files for tcp, http, and https
service_profile sections, it is worth explicitly testing each
section type.

This adds the missing tests for tcp service_profile sections:

* valid config
* a bogus config with a 'persistence' section specified.
Merge pull request #63 from alphagov/68445956_persistence_schema

[#68445956] Add missing Virtual Server persistence schema