Add support for Fedora 20

Since "Update to Fedora 20" (https://review.openstack.org/#/c/67505/)
was merged to diskimage-builder we should add support for Fedora
releases that are more than 19

Change-Id: Ic90d7380418ca5c3216aa3508930e607725ff177
Add an Hadoop CDH element

Add an Hadoop CDH element that installs only HDFS from CDH 4.5
Comments in the 40-setup-hadoop script indicate where code should be
added to support other CDH versions and add full support for the
distribution. If this version will be accepted, a bug report will be
created to track the missing features.

This element will be used to create an image with Spark, suitable for
use with the Savanna Spark plugin.

Change-Id: I6202e6f5d88cc697af53ff1fd5d23d35c80b2b6d
Add a Spark element

Add a Spark element that installs Spark in the image. By default the
script tries to find a suitable Spark version based on the Hadoop
version string. For now only the Hadoop CDH installed by the hadoop-cdh
element is supported.
Environment variables can be used to select a different Spark version or
use a custom Spark distribution, generated by hand.

This element is going to be used to build images that will be configured and
run by the Savanna Spark plugin.

Change-Id: Icd4dfef792a7c112bdcccc8951f7e86892abbe86
Sync with global requirements

There are still some problems with requirements updates proposal script,
so, syncing requirements manually.

* bump pbr to >=0.6,<1.0

Change-Id: If1caaf3bfffc3f76dfa94c9581251fa75601c050
Merge "Sync with global requirements"
Add qcow2 image files to gitignore

Change-Id: I4dd6562662cfc2b5480bdcc07a472ea688996727
Add options for Spark and Vanilla plugins

Add the '-p' commandline option to the diskimage-builder script to
select which Savanna plugin to target during image generation.
Vanilla and Spark plugins are supported. By default (if no option is
specified) the Vanilla plugin is selected.

Change-Id: I76af023ebd47bb026308b6034cacf01360739be1