dockerctl: Update authorized_keys in bootstrap images during restore
dockerctl: Update authorized_keys in bootstrap images during restore

After backup-reinstall-restore Fuel uses bootstrap with wrong ssh keys.

dockerctl backup procedure does not save the bootstrap's root
filesystem, which holds the list of authorized keys that should be
matching the private key located in /root/.ssh on the master.
It would result in inaccessible nodes from a new master installation
(which has bootstrap images regenerated with a new key, not matching
the original one, restored from the backup).

This patch updates authorized_keys inside squashfsed bootstrap images,
so it will be retained during backup-reinstall-restore procedure, if
user has not requested a full backup. Full backups already contain a
full copy of /var/www/nailgun, so no changes are needed.

User should carefully restore from non-full backups - only active
bootstrap will have its ssh keys updated. To propagate ssh keys to
already running nodes, you can use something like:
  mco rpc --agent execute_shell_command --action execute --arg cmd="echo '$(cat /root/.ssh/id_rsa.pub)' >> /root/.ssh/authorized_keys" -v

Change-Id: I3945200d02b29ed10e60900f7d03ed30970159a3
Closes-Bug: #1536198
DocImpact

stackforge/fuel-library,xarses/fuel-library,xarses/fuel-library,stackforge/fuel-library,stackforge/fuel-library,xarses/fuel-library,stackforge/fuel-library,xarses/fuel-library