Support setting passenger_pool_idle_time

[treydock]

Fixes #1208

[johrstrom]

Thanks! needs to be added to the example too though.

[treydock]

I updated the example config, also made sure to mention that setting false can avoid putting this in the NGINX configs.

[treydock]

I missed something, which e2e tests picked up. Running locally I get this when starting PUN:

Error -- undefined local variable or method `passenger_pool_idle_time' for #<NginxStage::PunConfigGenerator:0x0000561c34aa7520>
Did you mean?  passenger_root
Run 'nginx_stage --help' to see a full list of available command line options.

[johrstrom]

LGTM