Idempotent default server removal

When using the flag nginx_set_default_server, a default.conf file is written that returns a 404 on everything. But due to the preceding "state-absent" task, this was not idempotent, and would result in
remove and add on every provisioning run.

SSL improvements

• Loosen SSL settings a bit (for old browsers), see https://mozilla.github.io/server-side-tls/ssl-config-generator/
• Replace nginx_add_headers (a list) with nginx_http_headers (a dict)
• nginx_http_headers is empty, use to easily override entries in nginx_http_headers_default

Separated add_header default variable

Merge pull request #5 from f500/headers

Extract headers into dedicated variable so it can be reused

v2.0.3

Update meta from category to galaxy_tags

v2.0.2: Merge pull request #3 from damko/patch-2

Missing config line in the example Nginx config

v2.0.1: Merge pull request #2 from damko/master

Add Nginx example config for PHP7