Add parameter for osd recovery.

Signed-off-by: Jan Alexander Slabiak <j.slabiak@telekom.de>

To reduce impact of a node failure it's necessary to to configure
the osd_max_backfills and osd_recovery_max_active.

Add documentation and verification of the input of this parameters.
SMBCAP-5774
Merge pull request #19 from TelekomCloud/feature/osd_recovery_options

Add parameter for osd recovery.