Make the 'org.nuxeo.distribution.name' property configurable with a Maven property

The distribution name will then be configured only at one place in the root pom.

This prop will be used:

• in the Nuxeo Server Tomcat assembly.
• in all the Nuxeo packages package.xml file.

Having this prop will also ease the changes needed for the LTS repository: replace only this prop from server to lts.