tools: Adds UserDefinedVMProperties validator to engine-config

Adds validator of UserDefinedVMProperties validator to engine-config to
ensure that only valid values will be filled in UserDefinedVMProperties.

Change-Id: I503c00fe005b35314af72564059242a95edb4aa6
Bug-Url: https://bugzilla.redhat.com/962666
Signed-off-by: Martin Perina <mperina@redhat.com>