Add pre-checks prior starting the migration

[schaefi]

Some conditions e.g the one explained in #197 could be identified easily prior running the migration process. If it's clear the migration will fail and we can easily detect this situation before reboot or run_migration it would be good to add this type of checks.

I suggest the following checks:

• Lookup /etc/zypp/repos.d for locations that are not remote (e.g CD mount)
• Lookup if any of the filesystems in fstab are using LUKS encryption

Please add to the list if you can think of more pre-conditions we could check easily

[schaefi]

This here is also related to #173, would be good if one of the checks is a config file validator. That should be very easy since the schema is already there