LU-11006 lnet: fix show peer yaml tree with no peer

LU-11006 lnet: fix show peer yaml tree with no peer

When no peer exists then the root created for the peer
yaml tree should be deleted. And lnetctl show peer
should not display anything.

Currently lnetctl peer show shows the root string "peer"
even when there is no peer. This create issues when
starting lnet using /etc/lnet.conf derived from the
existing configuration.

Lustre-change: https://review.whamcloud.com/32320
Lustre-commit: 40295e5ca3e5ed51c8236a2e641627d687b7d59c

Change-Id: I58a337233c4dcea80e78c9dfd6eaabab33766f04
Test-Parameters: trivial
Signed-off-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-on: https://review.whamcloud.com/32318
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>