Keystore reset/purge admin command

[mbartosch]

It should be possible to clear the CertNanny state for a specific keystore using an administrative command (instead of manually deleting files from the state directory).
Suggested use:

To reset the state files of keystore "mykeystore" referenced in foo.cfg:
certnanny --config foo.cfg purgestate mykeystore

Bonus points:

• introduce a command to purge all keystores (possibly require use of --force to confirm the action)
• introduce a "list" command to list all configured keystores, possibly with current certificate status/expiration date