test/ftl: wait till nbd is ready when loading JSON config

When creating the nbd, we wait until it's ready, but we didn't do
that when loading the configuration from JSON, which resulted in
sporadic IO failures, as the device hasn't been initialized yet. This
patch adds waitfornbd after each load_config call.

Change-Id: Id350ae7b1afab11f5f3fbd131d938dbd65a8cb15
Signed-off-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/459616


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Wojciech Malikowski <wojciech.malikowski@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Mateusz Kozlowski <mateusz.kozlowski@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>