fabrics: prevent 'nvme connect' to well-known discovery NQN

fabrics: prevent 'nvme connect' to well-known discovery NQN

Using 'nvme connect' to connect to the well-known discovery NQN
could result in a few problems with the current implementation:

1) It does not explicitly disconnect from the discovery subsystem
after usage, thereby leading to a spurious discovery controller
on the host.

2) Multiple 'nvme connect' invocations to the well-known discovery
NQN would result in duplicate discovery controllers on the host.

All this is already handled in 'nvme discover'. So it would be
better to avoid using 'nvme connect' to connect to the well-known
discovery NQN in the first place, and use 'nvme discover' instead
for the same.

Signed-off-by: Martin George <marting@netapp.com>