geli_groups vs. fstab

I was testing geli_groups with a setup similar to the example here:
https://reviews.freebsd.org/D12644

The entries in rc.conf only work if the devices are not also listed in /etc/
fstab. The rc-script processes fstab entries before trying to attach the
geli_groups.

As workaround, I've edited /etc/rc.d/geli and exchanged the loops
"for group in ${geli_groups}; do" and "for provider in ${devices}; do".

Now it works as expected for me.

The commit message says:
    This is helpful when the providers being attached are not used for boot,
    and therefore the existing code to first try the cached password when
    tasting the providers during boot does not apply.

I'm not sure how the "cached password" mechanism works. My rc-change might
break it.

Disclaimer: Tested on 13.0-RELEASE, but the rc-script ist the same in in
current.