add toplevel folder to distribution zip

[weberjn]

If the openhab.zip would contain a toplevel folder, you'd just have to unzip it and be ready to go. Some beginners might not know unzip -d ..

In Maven speak this would be convention over configuration, as most open source software zips do contain a toplevel folder, e.g. Apache Tomcat, Wildfly or Eclipse.

In the Tomcat way, for openhab-3.0.1.zip the toplevel folder should be openhab-3.0.1

As for the pros who want a different folder name, they certainly know how to rename it.

[wborn]

In the Tomcat way, for openhab-3.0.1.zip the toplevel folder should be openhab-3.0.1

It will probably cause issues when updating the version number of an existing instance after an upgrade, see #860.