config: Always use osd_memory_target if set

config: Always use osd_memory_target if set

The osd_memory_target variable was only used if it was higher than the
calculated value based on the number of OSDs. This is changed to always
use the value if it is set in the configuration. This allows this value
to be intentionally set lower so that it does not have to be changed
when more OSDs are added later.

Signed-off-by: Gaudenz Steinlin <gaudenz.steinlin@cloudscale.ch>
(cherry picked from commit 4d1fdd2b05d55f8028fb5593d41fa61dbddd7095)