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.

Change-Id: Ie310a49e60386b579b48898b032467b1bc112da9
Test-Parameters: trivial
Signed-off-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-on: https://review.whamcloud.com/32320
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>