It should be possible to set the username/password via a header/property for the servicemix-mail provider

The main use-case for this is to actually avoid putting the username/password in the xbean.xml and instead obtain the username/password from an encrypted store and dynamically set these to be used by the servicemix-mail provider when it connects to the SMTP server. Currently the username, password, host and port are only read when the SU is deployed and it appears there's no header or property that the servicemix-mail provider checks before connecting to SMTP and sending the message.

This patch adds this functionality, also removes a little duplicate code.