Release 0.1.20

- add tar as an output type
- yum: support pkg-map in bin/install-packages
- rename old image file instead of rewrite it
- reinstate Trusty as default for Ubuntu
- only use Ec2 cloud-init data source for Ubuntu (bug 1316475)
- check return code of element-info
- correct the wrong rename in rhel element
- add new cloud-init-datasources element (bug 1316475)
- tidy up SuSE kernel selection
- parameterise PXE kernel and initrd selection
- debian: Support additional debootstrap arguments