LU-18680 ptlrpc: improve nodemap lproc syntaxes

LU-18680 ptlrpc: improve nodemap lproc syntaxes

- 'idmap='

Fix bad YAML syntax when only gid or projid were set, but no uid mapping.
Now simply a pair of brackets when empty, no more multiple or empty lines.

  idmap=[]

- 'exports='

Now each entry is on a separate line, likewise 'idmap'. While still
YAML compliant, this is easier to read for admins. The final ',' has been
cleared.
Simply a pair of brackets when empty, no more multiple or empty lines.

  exports=[]
  exports=[
   { nid: 172.16.0.1@tcp, uuid: 1d49406a-68eb-4d54-ae08-3587d6a6b078 },
   { nid: 172.16.0.1@tcp, uuid: 48ed3108-de34-11ef-bd15-670a7bd749aa }
  ]

- 'ranges='

Now simply a pair of brackets when empty, no more multiple or empty lines.
Still one entry per line when populated.

  ranges=[]

Change-Id: Ib1711614a825dc3bbb2b8861a61461fdea4e4f4b
Signed-off-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57915
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>