rbd: New parameter key-secret

rbd: New parameter key-secret

Legacy -drive supports "password-secret" parameter that isn't
available with -blockdev / blockdev-add.  That's because we backed out
our first try to provide it there due to interface design doubts, in
commit 577d8c9a811, v2.9.0.

This is the second try.  It brings back the parameter, except it's
named "key-secret" now.

Let's review our reasons for backing out the first try, as stated in
the commit message:

    * BlockdevOptionsRbd member @password-secret isn't actually a
      password, it's a key generated by Ceph.

Addressed by the rename.

    * We're not sure where member @password-secret belongs (see the
      previous commit).

See previous commit.

    * How @password-secret interacts with settings from a configuration
      file specified with @conf is undocumented.

Not actually true, the documentation for @conf says "Values in the
configuration file will be overridden by options specified via QAPI",
and we've tested this.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>