Fc.agent

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- Omit deleted nodes from generated backy configuration (13456).
- Include explicit values for pool and image in generated backy configuration
(13456).

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- backy config: generate valid config file even when there are no jobs present
(13456).
- Generalize bacula purge code, rename localconfig-bacula-purge-stamps to
localconfig-bacula-purge (13456).

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- localconfig-backy writes backy.conf from directory ENC data (13456).

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- Delete unused pools (no RG or RG contains no VMs) (14997).

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- Mark maintenance jobs as SUCCESS (but don't run the script) is `applicable`
returns with exit code 1 (16413).

- Fix bug which could leave pgp_num permanntly lower than pg_num (16711).

- Refactor maintenance: introduce global prepare/finish scripts that
can be run when maintenance starts or finishes. This allows evacuating
KVM hosts or outmigrating Ceph OSDs before starting maintenance,
rebooting, etc. This decouples the jobs' awareness of its environment -
maitenance jobs can expect that a machine may get interrupted. The prepare
and finish scripts are required to be careful to stop maintenance and
put things back in order when maintenance is finished (13755).

- Protect against users re-registering local Unix usernames (like root, apache,
or others) without us noticing. Currently you could still register them but
things would break on your machines anyway.

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- Automatic Kibana configuration (16002).
- Parallelize localconfig-kvm-init (14920).