Add a configuration file

[RKrahl]

There are several things that need configuration in the script:

• mail configuration: mail host, from, to,
• control of retrying if the ZYPP library is locked, added in #4: lock_max_tries and lock_wait,
• logging configuration, added in #5.

All of this is currently hard coded in the script, but should be made configurable in a configuration file.