If `osd_memory_target` is set in group_vars, the default value (4Gb)
should be overridden.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2118544

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 03713070eb481029d4493db0a565a9e753f059c9)