crowbar-prep.sh: add --media-mirror option

Since @saschpe sneakily changed the default to match his preferences ;-)
crowbar-prep.sh: move constants to beginning of script
crowbar-prep.sh: parametrize SP3 media NFS export subdirectory name
crowbar-prep.sh: parametrize SP3 ISO name
crowbar-prep.sh: add optional support for HAE ISO
crowbar-prep.sh: assign some variables after option parsing

In particular, variables depending on CLOUD_VERSION should be
set after the CLI arguments are parsed.
crowbar-prep.sh: assign static strings in the same place
crowbar-prep.sh: avoid hardcoding repo name in multiple places
crowbar-prep.sh: remove preexisting any Devel:Cloud:Shared repo
crowbar-prep.sh: Devel:Cloud:Shared only used for SUSE Cloud 3
crowbar-prep.sh: fix ibs_repo bug

Previously only worked if -p was specified before -d or -s, not after.
crowbar-prep.sh: avoid blank line leakage from multiple runs
crowbar-prep.sh: add missing dependency of D:C:3:S on D:C:3

Devel:Cloud:3:Staging only provides a subset of Devel:Cloud:3, so
-s requires adding Devel:Cloud:3 too.
Merge pull request #27 from aspiers/master

more misc enhancements