With this element other elements can register source code repositories by
placing their details in the file source-repository-\*. An example
of an element "custom-element" that wants to retrieve the ironic source
from git and pbr from a tarball would be

*File : elements/custom-element/source-repository-ironic*

    #<name> <type> <destination> <location> [<ref>]
    # <ref> defaults to master if not specified
    ironic git /usr/local/ironic git://github.com/openstack/ironic.git

*File : elements/custom-element/source-repository-pbr*

    pbr tar /usr/local/pbr http://tarballs.openstack.org/pbr/pbr-master.tar.gz

diskimage-builder will then retrieve the sources specified and place them
at the directory \<destination\>

A number of environment variables can be set by the process calling
diskimage-builder which can change the details registered by the element, these are

    DIB_REPOTYPE_<name>     : change the registered type
    DIB_REPOLOCATION_<name> : change the registered location
    DIB_REPOREF_<name>      : change the registered reference

for example if you would like diskimage-builder to get ironic from a local
mirror you could set DIB_REPOLOCATION_ironic=git://localgitserver/ironic.git

Alternatively if you would like to use the keystone element and build an image with
keystone from a stable branch then you would set DIB_REPOREF_keystone=stable/grizzly

Git sources will be cloned to \<destination\>

Tarballs will be extracted to \<destination\>. Tarballs should contain a
single topleval directory, regardless of the name of this top level directory
it will be renamed to \<destination\>

If mulitple elements register a source location with the same <destination>
then source-repositories will exit with an error. Care should therefore be taken
to only use elements together that download source to different locations.