IVGCVSW-4519 Remove Boost Variant and apply_visitor variant

IVGCVSW-4519 Remove Boost Variant and apply_visitor variant

  * replace boost::variant with mapbox::util::variant
  * replace boost::apply_visitor with mapbox::util::apply_visitor
  * replace boost::get with mapbox::util::get

Signed-off-by: James Ward <james.ward@arm.com>
Change-Id: I38460cabbcd5e56d4d61151bfe3dcb5681ce696e