fuel_version in metadata should be a list, not a string

A single plugin can be compatible with several versions
of fuel, it's the reason why fuel_version should be
an array of strings.

Change-Id: I42b462562fa6d112023c98e8f8cba7add6900608
Implements: blueprint cinder-neutron-plugins-in-fuel
Merge "fuel_version in metadata should be a list, not a string"