do not remove apache during nginx setup

Currently when you choose the nginx installation, Apache will be completely removed:
https://github.com/petrockblog/OwncloudPie/blob/b7849d2a494e19f5b82d2ac5d66dc387a2af7b0f/owncloudpie_setup.sh#L188

This doesn’t happen the other way around with the Apache installation. It also seems wrong, especially when Apache was installed for other apps, for example Wordpress (as it’s done in the [official documentation](http://www.raspberrypi.org/documentation/usage/wordpress/README.md)).

@petrockblog what do you think?