qa: add qa/tasks/nvmeof.py and rbd/nvmeof_basic_task and fio workunits

qa: add qa/tasks/nvmeof.py and rbd/nvmeof_basic_task and fio workunits

This is v2 of the rbd/nvmeof test: It deploys 1 gateway and 1 initiator.
Then does basic verification on nvme commands and runs fio.

This commit creates:
1. qa/tasks/nvmeof.py: adds a new 'Nvmeof' task which deploys
    the gateway and shares config with the initiator hosts.
    Sharing config was previously done by 'nvmeof_gateway_cfg' task
    in qa/tasks/cephadm.py (that task is removed in this commit).
2. qa/workunits/rbd/nvmeof_basic_tests.sh:
    Runs nvme commands (discovery, connect, connect-all, disconnect-all,
    and list-subsys) and does basic verification of the output.
3. qa/workunits/rbd/nvmeof_fio_test.sh:
    Runs fio command. Also runs iostat in parallel if IOSTAT_INTERVAL
    variable is set. This variable configures the delay between each iostat
    print.

nvmeof-cli upgrade from v0.0.6 to v0.0.7 introduced major changes
to all nvmeof commands. This commit changes v0.0.6 commands to
v0.0.7 in qa/workunits/rbd/nvmeof_initiator.sh

Signed-off-by: Vallari Agrawal <val.agl002@gmail.com>