Change dependency on callmgr.service

Change dependency on callmgr.service

Due to a bug[1] in systemd missing callmgr.service didn't prevent
lbs-server.service from starting. The bug was fixed in systemd 254 and
lbs-server does not start whith the hard dependency on callmgr. Changing
the dependency type from Requires to Wants makes lbs-server start properly
under the new systemd.

[1] https://github.com/systemd/systemd/issues/11338

Change-Id: I4e3df83f1afa9ed983d6af7b2f9cb5802bd7b2d0
Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>