CMS: create a new 2015 VM page

[katilp]

Create a new page for VM and 2015 data docs/cms-virtual-machine-2015 as in http://opendata.cern.ch/docs/cms-virtual-machine-2011

To be changed, compared to the 2011 version:

• title could be "CMS Virtual Machines: How to install for 2015 data" instead of "CMS 2011 Virtual Machines: How to install"
• references to 2011-2012 CMS Open Data -> 2015 CMS Open Data
• CMSSW version to CMSSW_7_6_7
• add mention that the same image can be used with the 2011-2012 data but with a different CMSSW version

The image can stay the same (http://opendata.cern.ch/record/252) but its title may need updating, now "CMS VM Image, for 2011 and 2012 CMS open data", or at least 2015 should be mentioned in the description

Would have been better to name it after OS...

[katilp]

NB cernopendata/data-curation#116
mkedanlzr does not work out of the box, replace that with edmDumpEventContent