Fix assembly warnings raised by Ant Assembly Maven plugin

Since the move from nuxeo-distribution-tools to ant-assembly-maven-plugin, the original assemblies are raising warnings to fix.
For instance, on nuxeo-distribution-tomcat:

• [WARNING] [echo] Building Tomcat standard distributions...
• [WARNING] The classifier parameter is deprecated, put it in the key.
• [WARNING] [artifact:attach] The target parameter is deprecated and ignored. The attach task now only applies to the current project.

See changes list on NXP-13555