Implement serial-console for systemd.

Update install.d/20-stty so that we try to start
a getty on ttyS0 and ttyS1 when using systemd.

Change-Id: Id5b605c74bf82855af0af4d0f95659cdd3a7b3a0
Wait for tgtd socket to be available

tgtd returns execution control and backgrounds itself almost immediately
and before it has made it's listening socket available. This can cause a
race condition as the tgtd socket is not available when tgtadm is run,
resulting in an error:

failed to send request hdr to tgt daemon

Add a function to check if the socket is available before moving on to
calling tgtadm, and a wait_for helper function we can use.  We'll
check for the socket every 0.5 seconds, for up to 5 seconds.

I'm seeing this issue on almost every deploy using a ramdisk built from
Fedora 20. I'm not sure if something has changed in tgtd, but this
behavior is documented since Fedora 18 at least. In the systemd script
for tgtd, there is actually "sleep 5" to work around the problem.

See Also: https://bugzilla.redhat.com/show_bug.cgi?id=848942

Change-Id: Iffa9fc63393309ca653d592dff17316ecbea3e09
Add support for Debian

Change-Id: Ibc6bc57dea1eab295fb23cccf40b38f3fcb285c3
dib-run-parts should dereference symlinks

dib-run-parts should dereference symlinks, and if they point to an
executable file, execute that file as a script. This can be accomplished
by using the xtype predicate in the find command instead of the type
predicate.

This change is needed so that we can dynamically symlink hook scripts
into hook directories such as install.d at runtime to support different
install types.

Change-Id: I933e7f4b4dcf16956841d8c14aa63b0f9a18fc5d
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
Merge "Implement serial-console for systemd."
Merge "Wait for tgtd socket to be available"
Merge "dib-run-parts should dereference symlinks"
Merge "Symlink correct element install type"
Updated rhel element to support rhn

Added support for Red Hat Network (RHN) subscription. Extended to accept list of rhn channels or repos to enable. Default subscription method remains rhsm for backward compatibility. Also updated the default RHEL image to the latest RHEL 6.5 image available on the Customer Portal.

Change-Id: Ia7f39bee7ae24253fa2002b3dc5b2293be3ff0bd