Add dry-run mode

[TobiPeterG]

I'm one of the contributors to https://github.com/openSUSE/sdbootutil, where we handle systemd-boot on openSUSE based distributions. One issue we have is that we are unable to detect if a rebuilt of the initrd is required and we reuse existing initrds as often as possible as generating an initrd is an expensive operation. This leads to issues, as a user might install a module that doesn't get automatically included in the initrd.

We would love to see a "dry-run" mode in which dracut only checks the configs and modules to find out if the modules/configs have changed since the last build. No initrd should be created, it should just be able to check if a rebuild is required.

This is related to openSUSE/sdbootutil#22

[LaszloGombos]

CC @aafeijoo-suse

[aplanas]

@TobiPeterG maybe this can help too: openSUSE/suse-module-tools#103