Drop support for "kvmibm" distro

The IBM hypervisor distro "KVM for IBM z Systems" gets discontiued,
like announced in March 2017 [1]. The key dates are:

* 03/2017: announcement
* 08/2017: the last day to order (EOM)
* 03/2018: the End of Service (EOL)

As the CI which tests OpenStack with KVM on IBM Z doesn't rely on this
distro anymore and EOM has reached, we remove the Devstack support for
this distro.

This basically reverts commit a5ea08b of Dec 2015.

NOTE: This doesn't affect other distros which have KVM on Z support.

References:
[1] FAQ for KVM for IBM z Systems Delivery Strategy Change
    https://www-01.ibm.com/common/ssi/cgi-bin/ssialias?htmlfid=ZSQ03110USEN&

Change-Id: I009ae4779588615633bff81d0c47a1b879ec9279
diff --git a/stack.sh b/stack.sh
index c545c56..8632cf1 100755
--- a/stack.sh
+++ b/stack.sh
@@ -221,7 +221,7 @@
 
 # Warn users who aren't on an explicitly supported distro, but allow them to
 # override check and attempt installation with ``FORCE=yes ./stack``
-if [[ ! ${DISTRO} =~ (xenial|yakkety|zesty|stretch|jessie|f24|f25|f26|opensuse-42.2|opensuse-42.3|rhel7|kvmibm1) ]]; then
+if [[ ! ${DISTRO} =~ (xenial|yakkety|zesty|stretch|jessie|f24|f25|f26|opensuse-42.2|opensuse-42.3|rhel7) ]]; then
     echo "WARNING: this script has not been tested on $DISTRO"
     if [[ "$FORCE" != "yes" ]]; then
         die $LINENO "If you wish to run this script anyway run with FORCE=yes"