This is an update for diskimage-builder to version 2.15.1 including the following fixes/features:

Version 2.15.1 (boo#1097115):
- elements: pip-and-virtualenv: Handle openSUSE Leap 15
- Allow to rebuild arbitrary images
- Replace the ubuntu-minimal trusty test with a bionic one
- Remove non-maintained ubuntu-core element
- elements: zypper-minimal: Add support for openSUSE Leap 15.X
- Add support for Ubuntu 18.04
- Bugfix: Remove duplicate GRUB command line entry
- rpm-distro: set the contentdir yum var
- Fixes add-apt-keys in dpkg element
- Add pip cache cleanup to pip-and-virtualenv
- pip-and-virtualenv: fix install-pip when centos-release-openstack is enabled