Unbelieveable how annoying this RHEL 7 installation wizard is! Try to setup manual partitioning with a mix of primary partitions and logical volumes - and RHEL will not boot. Even worse: Although I created the root partition first, somehow the wizard switched it with the swap partition making swap on /dev/sda1 instead of /dev/sda2 (where I wanted it to be).
Solution to this: Boot from Knoppix (yes, seriously) and manually create the partitions with parted. Once this was done, rebooted from the RHEL image. On the "Installation Destination" submenu I selected "I will configure partitioning" and then clicked on the blue Done button.
Inside the "Manual Partitioning" window I clicked on the rescan button and the disk was rescanned. The already created partitions were shown under a section called "unknown". To tell the RHEL7 installer to use these partitions, I had to select each partition and enter the mount point and click on "Reformat". The partitions were then finally shown as I wanted them in the installer:
This is surely the worst partitioning wizard I have seen in a recent Linux distribution (and I installed new machines with Ubuntu 16.04, SLES 12, CentOS 7, Debian Jessie and Linux Mint 17.3 in the past months).