Support Nexus 2.7

Nexus 2.7 was released and we should support it.

I also want to remove some of the code that requires a data bag, to simply the basic "clone and vagrant up" workflow. Plus we should only install a Nexus server. If someone wants to configure it, thats what a wrapper cookbook / using the LWRPs is for.

Some of the changes in this pull might render this cookbook unuseable on older versions of Nexus. In particular:

* the jetty.xml changed in Nexus 2.7
* the nexus binary changed in Nexus 2.7 - I was also required to forward port a change I made to make the service work with Chef - (see 5aa740c5b7e11c22ce9fc8c96061a4dc1f73a798)