Options/Common: remove defaults for ssl-key/ssl-cert on the Client

With these default values it is impossible to remove the ssl key and ssl cert
from the configuration and use password auth. Configuration options could not
be condensed in Bcfg2.Options.Common, because Server and Client needs
different default values.
Merge pull request #209 from AlexanderS/fix-password-auth

Options/Commmon: remove default values for ssl-key/ssl-cert