crowbar-prep.sh: split common_post into smaller functions for legibility
crowbar-prep.sh: Devel:Cloud:Shared is needed for Cloud 4 too
crowbar-prep.sh: allow overriding of defaults

This allows people to put stuff like

  export HOST_MEDIA_MIRROR_DEFAULT=/mnt
  export HOST_MIRROR_DEFAULT=/srv/mirrors

in their shell profile, and then still be able to override via the
command-line.  Note that due to the cat crowbar-prep.sh | ssh ...
style of invocation, the export would have to go in the shell profile
on the admin node, not the node where crowbar-prep.sh is available.
So it may still be easier to use the -m and -n options.
crowbar-prep.sh: fix usage output order
crowbar-prep.sh: add -l for local mirrors of Devel:Cloud:* IBS repos
crowbar-prep.sh: add SLE11-HAE-SP3-{Pool,Updates}