LU-17184 mgc: remove damaged local configs

LU-17184 mgc: remove damaged local configs

If local config llog is damaged it can't be removed and
prevents target from mounting. This happens because
mgc_llog_local_copy() uses llog_erase() to remove llogs
which can't do the job if llog header is damaged.

Patch changes are:
- llog_erase() to don't initialize header but just destroy
  llog file
- mgc_llog_local_copy() to don't exit on backup to temp
  file but continue with remote llog copying anyway
- conf-sanity test_151 is added to check that target can
  mount with damaged local config

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I637749c38fd5ed03bdac5ca1cd60196f724ab0d1
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52697
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>