Allow source-repositories to be disabled completely

In our official image builds we are only allowed to use resources
that are "blessed" by the build system.  This means that external
things like git repos and tar files are not allowed.  Currently,
even in offline mode source-repositories expects those things to
be available in the cache, so we need a way to disable it entirely.

This change adds an environment variable NO_SOURCE_REPOSITORIES
that does so.  It can be set in an environment.d script so elements
that might rely on a source repository will know it's not available.
The 02-lsb script in redhat-common is one such example and is
updated to handle this case.

Change-Id: I0de63bee6ad79733d6711478c707a9b41593e85f
Avoid overwritting of hooks

Until now there was a possibility for two elements to install hooks
with the same name, so one of them was overwritten. Change logic to
copy the hooks and fail in case one with the same name exists.

Change-Id: Ic2c46835b27c9319f7a889ffd0ccf3f5ccc1f0cd
Closes-Bug: 1251952
Enable dracut deploy ramdisks

The element builds dracut from source on Ubuntu because the
Ubuntu dracut package is broken and very old, so it can't be
installed properly and causes a number of other issues that
are fixed by using a newer version of Dracut.

This initial version should work in virtualized environments.
Further validation of its suitability for real baremetal
deployments will need to be done in the future, but this should
be sufficient to enable that work.

Regarding Dracut specifically, in order to limit the changes
needed in the existing scripts this element continues to use a
cut down version of the /init script that we were building for the
existing ramdisk.  However, instead of running it as pid 0 it is
run as a Dracut pre-mount hook.  This allows Dracut to set up all
of the hardware and system bits, while falling early enough in the
Dracut sequence to complete the deployment before Dracut would try
to boot off the hard disk.

bp tripleo-juno-dracut-ramdisks
Change-Id: I144c8993fe040169f440bd4f7a428fdbe3d745cf
Merge "Enable dracut deploy ramdisks"
Merge "Avoid overwritting of hooks"
Merge "Allow source-repositories to be disabled completely"
iso element to build bootable ISO images

This commit adds a new element named 'iso' to build a bootable
ISO image for the kernel/ramdisk emitted by the 'baremetal' or
'ramdisk' element.

Change-Id: I89d175a29e2d0bc64b47fe527f0d0f6875f6849a