Add support for optional collector fields and (null expr) test

This adds support for fields that might or might not be
collected. When that happens the data fields are set to None.

The policy can then use (valid expression) to test whether
expression evaluates to non-None value.

Change-Id: I3a93f0985870b431c315918b14fee6629eac70fb
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21050
Reviewed-by: Adam Litke <agl@us.ibm.com>