nvme-sysfs: add 'tls_configured_key' sysfs attribute

There is a difference between the negotiated TLS key (which is
always present for a TLS encrypted connection) and the configured
TLS key (which is specified with the --tls_key command line option).
To differentate between these two add a new sysfs attribute
'tls_configured_key' to hold the specified on the command line.

Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>