spec: simplify docker-storage-setup install
Now that all of the platforms default to overlay2 on fresh installs, we
can simplify the spec file a lot wrt. docker-storage-setup.
Note the 'STORAGE_DRIVER=overlay2' is already injected at %install time
by `make install-docker`.
This should work fine when upgrading installs with devicemapper since
c-s-s now exits nicely if /etc/sysconfig/docker-storage already exists
and specifies a different driver.
The motivation for this patch is the fact that the `%posttrans` is not
guaranteed to be run after `/etc/os-release` is ready depending on the
RPM transaction ordering. See also:
Based originally on a patch from:
Colin Walters <firstname.lastname@example.org>