init-radosgw.sysv: Support systemd for starting the gateway

When using RHEL7 the radosgw daemon needs to start under systemd.

Check for systemd running on PID 1. If it is then start
the daemon using: systemd-run -r <cmd>. pidof returns null
as it is executed too quickly, adding one second of sleep and
script reports startup correctly.

Signed-off-by: JuanJose 'JJ' Galvez <jgalvez@redhat.com>
Merge pull request #2486 from jgalvez/master

init-radosgw.sysv: Support systemd for starting the gateway

Reviewed-by: Sage Weil <sage@redhat.com>