Run web UI using `-ui` and remove dependency on nginx

[colin-nolan]

An nginx server to run a separate web UI has not been required since the -ui option was added to Consul in 0.7.x. In addition, as of Consul 0.9.0, a web UI is no longer going to be released separately:
https://www.consul.io/docs/upgrade-specific.html#web-ui-is-no-longer-released-separately.

Although the consul_enable_nginx_config variable allows the install and setup of nginx to be avoided, I believe it is there so people can use a pre-existing nginx sever. There is no ability to pass -ui when starting the agent: https://github.com/savagegus/ansible-consul/blob/master/templates/consul.systemd.j2#L13.

It would be good to allow the UI to be ran via the agent, without nginx. This is not only desirable in 0.7+, but required in 0.9+. Going forwards, I feel that we should drop support for Consul < 0.7, which would allow the removal of the complexity of setting up and configuring nginx.