role nginx_reverse_proxy: refactor into generic nginx role

[dometto]

The role is basically usable for configuring nginx location blocks of any kind, not just reverse proxies. However, at present some reverse proxy directives are added out of the box that are not needed and create noise when using the role to create a normal location block.

[dometto]

Currently set_proxy_header and add_headers are special keywords that are mapped into many different set_proxy_header and add_header directives. We could generalize this so that you can pass in arbitrary dicts into the settings, so that a dict named foo would generate a directive foo first_key first_val; for each of its key/value pairs.