WIP : Feature flags

More context : 
https://one.rackspace.com/display/GDP/Phpstack+feature+flags+integration

@cybermerc @martinb3 @siso @fernandohonig . I've already worked on some features. It might be a good start to see what's good what's bad. A lot of things can be done differently, let's all find the best way to work on feature flags :smile: .

Enabled feature flags : 
* phpstack:flags:monitoring:enabled
* phpstack:flags:monitoring:newrelic
* phpstack:flags:monitoring:cloud-monitoring
* phpstack:flags:webserver:enabled
* phpstack:flags:webserver:apache
* phpstack:flags:webserver:nginx
* phpstack:flags:application:enabled

You can see in Specs it highlight some dependencies between attributes (by example you need to enable the demo in order to test apache etc...).
I've moved some attributes to attributes files in order to test it more easily
I've tried to make it converge with Kitchen+Role but it's failing (I didn't have time to check why)
I've not run rake style yet.
Specs are telling this "story" : 
```
phpstack::feature-flags
  if Webserver is disabled(default)
    doesn't includes the nginx recipe
    doesn't includes the apache recipe
  if Application deployment is disabled(default)
    doesn't includes the application recipe
  if Application deployment is enabled
    includes the application recipe
  if Webserver is enabled
    and apache is enabled
      includes the apache recipe
      doesn't includes the nginx recipe
    and nginx is enabled
      includes the nginx recipe
      doesn't includes the apache recipe
  if Monitoring is enabled
    and Newrelic is enabled
      includes the Newrelic recipe
    and cloud monitoring is enabled
      when we install nginx
        deploys cloud-monitoring configuration for HTTP
      when we install apache
        deploys cloud-monitoring configuration for HTTP
    and cloud monitoring is disabled
      when we install nginx
        doesn't deploy cloud-monitoring configuration for HTTP
      when we install apache
        doesn't deploy cloud-monitoring configuration for HTTP

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13 examples, 0 failures
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