Add option to prompt for keystore password instead of saving the hash in vault.properties

[csutherl]

The current version stores almost all of the pieces necessary to decode the keystore password in use with the vault. Should we provide users with an option to store none of it and require the password to be entered at runtime?

[csutherl]

Duplicate of #44 I think. This is slightly different, but I don't think it's a good idea anymore.

[csutherl]

Reopening this. I think we should add a feature like httpd's SSLPassPhraseDialog. Thoughts?