install: Partition probing ain't reliable

It seems that the partition probing during the installation wasn't
reliable, an extra call was added to flush all partition informations.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1156112
Change-Id: If889041a917112a06534595ba486ad473c6b7f4a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1150377
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>