installer: Be more conservative with partprobe

Previously partprobe was called in many places to update the partition
device links.
The initial cause for the missing device links were problems with udev
and multipath.
Now that these bugs have been addressed, the partprobe usage is known to
be buggy when layered dm devices exist or something is mounted. Thus we
remove the partprobe usage to reduce the probability for problems caused
by an excessive use.

Change-Id: I713e83009e6c623078ef30ffdbd0a620515cf8de
BNug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1167240
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
recipe: More robust multipath configuration

Previously some multipath configuration snippet got appended to the
existing configuration file.
This could lead to a situation where the defaults section appeared
twice, this made the configuration invalid, and multipath would not work
at all.

Now a different approach is used to ensure that a vlid configuration is
written.

Change-Id: I1e878b8cd2ce4b456380300dca2fa51c92523547
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>