Add parameters support to OVA plugin

Define a list of parameters and pass the correct subset to either vSphere or RHEV-M
package generator class.
Rename ovf_desc parameter

Parameters common to both rhevm and vsphere ova
will be named ovf_*.

This parameter is specific to the rhevm ova generation, so
name it rhevm_description.
Accept parameters for rhevm ova generation

Accept defined parameters with defaults, and use them when generating
the ovf metadata xml.
Accept parameters for vsphere ova generation

Accept defined parameters with defaults, and use them when generating
the ovf metadata xml.
Merge pull request #308 from eggmaster/ova_parameters

Make OVA target accept parameters