Refactor HTTP Proxy module

[alessfg]

Proposed changes

Refactor the proxy HTTP config template into its own separate file. All variables have changed (check defaults/main/template.yml for examples):

• All proxy_* related variables now live under the proxy dictionary key, with the exception of proxy_pass and proxy_cache_path. You can specify the proxy dictionary key inside the http, server, and location contexts.
• Remove the nginx_config_main_template.http_settings.cache dictionary variable. Use nginx_config_http_template.*.proxy_cache_path instead.
• Remove the location.websocket variable. Use location.proxy.set_header instead:

proxy:
  set_header:
    - field: Upgrade
      value: $http_upgrade
    - field: Connection
      value: Upgrade

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have added Molecule tests that prove my fix is effective or that my feature works
• I have checked that all Molecule tests pass after adding my changes
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md)