Symlink correct element install type

The source-repositories element will now symlink the install scripts
corresponding to the set install type for an element into the install.d
directory.

Different install types are implemented by elements by writing scripts to do
the install type under install.d/<element-name>-<install-type>-install
directories.

For example, the nova element would provide:

nova/install.d/nova-package-install/74-nova
nova/install.d/nova-source-install/74-nova

source-repositories will create the following symlink for the package
install type:

$TMP_HOOKS_PATH/install.d/74-nova -> nova-package-install/74-nova

Or, for the source install type:

$TMP_HOOKS_PATH/install.d/74-nova -> nova-source-install/74-nova

Change-Id: I1bfaf39e5a98b2af904fbc6d674dbada30b27ed9
Update default RHEL guest image

Red Hat periodically updates the qcow2 guest image available
for download. This sets the default image name to the latest.

Change-Id: Iba3075bbee3b41918d5cd3da9721fcbf98ff3bcd
Add support Red Hat Network (RHN)

This provides basic support to register to the Red Hat Network
(RHN Classic) for packages. The image is unregistered from RHN
before completing image build. The rhnsd service is prevented
from starting since it is not needed for packagage install.

Change-Id: I2dc124cdc6ae59a63fd94d82d77108acccc3f668
Support list of Red Hat channels and repos

When registering to RHN or RHSM a list of space-separated
channels or repositories may be provided. This allows control
over which packages are available to the system during build.
Note, the optional channel or repository is added by default
since diskimage-builder requires access to its packages. This
list is for additional channels or repos.

Change-Id: I9cf90d47ab1d09f5491f574132410438f9e294e3
Add support for Red Hat Satellite

A Red Hat Satellite URL may be provided as the source for
Red Hat packages. A Satellite key may be provided for a pre-
defined set of RHN channels to be automatically applied. Support
for installation of a Satellite certificate also provided.

Change-Id: Iae5b97d6f4732a28efa08ab778a56c2f9b6b6bd9