User fixed versions of the plugins

Otherwise an old maven instance might cause different build results than a current one
(e.g. maven 3.0.5 by default uses an maven-war-plugin causing a warning regarding the
web.xml which can be avoided this way).
Fix NPE if no previous deployment exists

We cannot replace an older version if no older version exists.
Move bean validation after password validation

Otherwise the model does not yet have the necessary values set and bean validation
would always fail.
Add XSD for validating the deployment descriptor of a server
Allow multiple copy/exctract actions per project

Restricting them to only one is unnecessary.
Regenerate meta-model after changing to multiple actions
Remove obsolete workaround code