devtest_overcloud: No need to set NODE_ARCH twice

Updates the devtest_overcloud.sh script so we only
set NODE_ARCH once.

Looks like there was a mixup in:
 b9eb2e60cb0a00522f584286f8bd525d0d5fbbb1

Change-Id: I7336e0ba8789d0c003505586223410d43927fbb8
Merge "devtest_overcloud: No need to set NODE_ARCH twice"