Add a pip-cache element.

The element bind mounts a pip cache inside the image build chroot so
that pip downloads can be reused across image builds.  While similar
in purpose to the PyPi element that sets up a mirror, this element
just allows for a reusable download cache and doesn't require anything
to be setup beforehand.

The pip-cache element is not concurrency safe, and that is indicated
in the README for the element.  An upstream bug was file as well:
https://github.com/pypa/pip/issues/1141

Change-Id: Ibd1d4ea17c24923ed939357ada95b781e3179cfd
Rename 51-grub to 51-bootloader

This patch is part of a another big patch that will implement a fallback
to Extlinux in case GRUB2 is not available in the system. The reason why
it's being splitted in two patches is because git is not being able to
recognize the file changes when the rename happens in the same commit.

Change-Id: Ic2027dfa057fd6d62b532203b7ff50c3c739bccb
Extlinux fallback

If GRUB2 is not available in the system a fallback to install Extlinux
will happen. This patch also allows you to enforce the use of Extlinux
exporting the DIB_EXTLINUX variable.

Change-Id: I88fa792a0bd19cacca167134f2054b11c807a451
Combine compress and save image into one function

When compressing an image, this is done in the same dir where the raw
image resides, doubling the amount of space needed (scarce when
using tmpfs), and then it's moved to the .cache folder in disk.

Combining these two functions, we reduce the amount of space needed
in the tmpfs partition (when in use), and the compressed image is
created directly on the .cache folder disk, so there is no need to
move the compressed image after the process into disk.

Change-Id: I451d24bdd6fa0983414244135dff5e96c0549833
Add package mappings for MySQL-MariaDB

Partial-Bug: #1213891

Change-Id: I1ac1d11ff1716eccee4c734395282be25dd5b541
Add option --min-tmpfs <size> to disk-image-create

In some scenarios, the required space in the tmpfs partition can be
larger (or smaller) than the default one, producing errors due to
the lack of enough space (or performance penalties for not using
tmpfs).

Using --min-tmpfs <size>, we can hint the working set size we'll need
and let dib choose to avoid or use tmpfs.

Change-Id: I7d5fe498302a100c8555ae542268e14b21f3a0c5
Add rhel installation element

If you want to have the installation update packages, you'll
need to register the system log in to rhn and subscribe to an
available subscription.
export DIB_RHSM_USER to your rhn username
export DIB_RHSM_PASSWORD to your rhn password

To get the qcow2 image, log into rhn.redhat.com and download the
image from
https://rhn.redhat.com/rhn/software/channel/downloads/Download.do?cid=16952
Then export DIB_CLOUD_IMAGES to whereever you're hosting the qcow2.

Change-Id: Idb547f4ffe75514b1e3f6b34f5f347493b132925
Merge "Combine compress and save image into one function"
Merge "Add package mappings for MySQL-MariaDB"
Merge "Add option --min-tmpfs <size> to disk-image-create"
Merge "Add rhel installation element"