Improve --ramdisk-element docs

Unlike most of the options to disk-image-create, this one didn't
document its default value, and it also didn't have any context for
why one might use it.

Change-Id: I57ea529cf387ab8ec8497d35be19ac64198df47a
Related-Bug: 1397073
Migrate to new package-installs

We have a new package-installs file format. Migrating existing
package-installs files to the new format.

Change-Id: I57d4a007efee9624e60c41357cefa627d8c7373f
Don't use lsb_release

Using lsb_release to figure out what distro and release an image is
is redundant, as the same information is available in DIB_ vars.
Further, it causes errors when creating new rhel7/centos7 images
like this: http://paste.openstack.org/show/144729/

Change-Id: I213ee581b8abfe119ffe3b783e6a3236e58e3e47
Allow injecting arbitrary yum repo configuration

Allows specifying the path to a yum repo configuration file to be used
during the image build process. The repo configuration file is removed
at the end of the build. This is useful in that it allows using any
arbitrary repo configuration during an image build.

Change-Id: I7d5c67d74a0bb4722ffc60aacfd9fa7e80fb59d5
Merge "Allow injecting arbitrary yum repo configuration"
Merge "Migrate to new package-installs"
Merge "Improve --ramdisk-element docs"