xCAT does not currently consider the RSTe array as a default install target. To override the behavior and direct the OS to be installed to RSTe, first create a file called /install/custom/sles12rste.partitions containing the following:

<drive>
 <device>/dev/md/Volume0</device>
 <partitions config:type="list">
     <partition>
         <filesystem config:type="symbol">vfat</filesystem>
          <mount>/boot/efi</mount><size>128mb</size>
      </partition>
      <partition>
          <mount>swap</mount>
          <size>auto</size>
      </partition>
          <partition>
           <mount>/</mount>
           <size>auto</size>
      </partition>
 </partitions>
 <initialize config:type="boolean">true</initialize>
</drive>

With this in place, modify the osimage to use this partition plan:

chdef -t osimage sles12.3-x86_64-install-compute partitionfile=/install/custom/sles12rste.partitions

From that point forward invocations of the nodeset command will target the RSTe volume.