test/nvmf: replace rpc_cmd() with direct invocation of rpc.py due

to inherently larger timeout

Fixes #3295

Change-Id: I7e8bce339d94ffd15aee6a0685c9eba981a754ab
Signed-off-by: Jaroslaw Chachulski <jaroslawx.chachulski@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/23391


Reviewed-by: Michal Berger <michal.berger@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Marek Chomnicki <marek.chomnicki@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>