[2.28.0] crashes with error: environment.d/51-pip-and-virutalenv-default.bash: line 5: GenericCloud: unbound variable

[Gudrutis]

We are running diskimage-builder on docker container based on CentOS7. However, version 2.28.0 is crashing when ran with parameters seen below. The 2.24.0 version on the same container is working fine.

```
> ELEMENTS_PATH=cci-elements DIB_REPOREF_ironic_agent=$IPA_BRANCH disk-image-create -o /ironic-image-creation/outputimage/$TEMP_FOLDER/$TEMP_FOLDER centos7 posix local-config disable-selinux cleanup-kernel-initrd ironic-agent ironic-hwmanager-cern ironic-rsyslog mellanox
...
2019-10-14 15:56:52.198 | ++ '[' -z '' ']'
2019-10-14 15:56:52.198 | ++ '[' centos7 == ubuntu ']'
2019-10-14 15:56:52.198 | ++ '[' centos7 == debian ']'
2019-10-14 15:56:52.198 | ++ '[' centos7 == fedora ']'
2019-10-14 15:56:52.198 | ++ '[' centos7 == centos7 ']'
2019-10-14 15:56:52.198 | ++ DIB_PYTHON_VERSION=2
2019-10-14 15:56:52.198 | ++ '[' -z 2 ']'
2019-10-14 15:56:52.198 | ++ export DIB_PYTHON_VERSION
2019-10-14 15:56:52.198 | ++ export DIB_PYTHON=python2
2019-10-14 15:56:52.198 | ++ DIB_PYTHON=python2
2019-10-14 15:56:52.198 | dib-run-parts Sourcing environment file /tmp/dib_build.HHRsKYaF/hooks/root.d/../environment.d/50-redhat-common
2019-10-14 15:56:52.201 | + source /tmp/dib_build.HHRsKYaF/hooks/root.d/../environment.d/50-redhat-common
2019-10-14 15:56:52.201 | ++ export DIB_DISABLE_KERNEL_CLEANUP=0
2019-10-14 15:56:52.201 | ++ DIB_DISABLE_KERNEL_CLEANUP=0
2019-10-14 15:56:52.201 | dib-run-parts Sourcing environment file /tmp/dib_build.HHRsKYaF/hooks/root.d/../environment.d/51-pip-and-virutalenv-default.bash
2019-10-14 15:56:52.204 | + source /tmp/dib_build.HHRsKYaF/hooks/root.d/../environment.d/51-pip-and-virutalenv-default.bash
2019-10-14 15:56:52.204 | ++ [[ centos7 =~ (centos|rhel) ]]
2019-10-14 15:56:52.204 | ++ [[ GenericCloud -ge 8 ]]
2019-10-14 15:56:52.204 | /tmp/dib_build.HHRsKYaF/hooks/root.d/../environment.d/51-pip-and-virutalenv-default.bash: line 5: GenericCloud: unbound variable
2019-10-14 15:56:52.513 | INFO diskimage_builder.block_device.blockdevice [-] State already cleaned - no way to do anything here
```

> rpm -q centos-release
centos-release-7-6.1810.2.el7.cern.x86_64

> pip show diskimage-builder
---
Metadata-Version: 2.1
Name: diskimage-builder
Version: 2.28.0
Summary: Golden Disk Image builder.
Home-page: https://docs.openstack.org/diskimage-builder/latest/
Author: OpenStack
Author-email: <email address hidden>
Installer: pip
License: Apache License (2.0)
Location: /usr/lib/python2.7/site-packages
Requires: Babel, PyYAML, networkx, stevedore, six, pbr, hacking
Classifiers:
  Development Status :: 5 - Production/Stable
  License :: OSI Approved :: Apache Software License
  Operating System :: POSIX :: Linux
  Programming Language :: Python :: 2.7
  Programming Language :: Python :: 3
  Environment :: Console
Entry-points:
  [console_scripts]
  dib-block-device = diskimage_builder.block_device.cmd:main
  disk-image-create = diskimage_builder.disk_image_create:main
  element-info = diskimage_builder.element_dependencies:main
  ramdisk-image-create = diskimage_builder.disk_image_create:main
  [diskimage_builder.block_device.plugin]
  fstab = diskimage_builder.block_device.level4.fstab:Fstab
  local_loop = diskimage_builder.block_device.level0.localloop:LocalLoop
  lvm = diskimage_builder.block_device.level1.lvm:LVMPlugin
  mkfs = diskimage_builder.block_device.level2.mkfs:Mkfs
  mount = diskimage_builder.block_device.level3.mount:Mount
  partitioning = diskimage_builder.block_device.level1.partitioning:Partitioning
  [diskimage_builder.block_device.plugin_test]
  test_a = diskimage_builder.block_device.tests.plugin.test_a:TestA
  test_b = diskimage_builder.block_device.tests.plugin.test_b:TestB

[Ian Wienand (iwienand)]

This should be fixed with https://review.opendev.org/#/c/688321/, which i'll release with 2.28.2