LU-14893 lctl: check user for changelog_deregister

LU-14893 lctl: check user for changelog_deregister

If no user is specified for "lctl changelog_deregister", usage
should be printed correctly.
Also, sanity.sh test_106e is modified to verify this fix.

Fixes: a15eb4f13224e ("LU-13055 mdd: per-user changelog names and mask")
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: Ia7f1b18e82f6b4174b9435cd67aba5f591d43ce1
Reviewed-on: https://review.whamcloud.com/44432
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>