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.