telemetry: add ability to disable selftest

telemetry: add ability to disable selftest

This patch adds functionality to enable/disable the selftest.

This functionality will be extended in future to make the
enabling/disabling more dynamic and remove this 'hardcoded' approach. We
are temporarily using this approach due to the design changes (vdev vs eal)
made to the library.

Signed-off-by: Ciara Power <ciara.power@intel.com>
Signed-off-by: Brian Archbold <brian.archbold@intel.com>
Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>