glusterd2.service: Read in env vars from /etc/sysconfig/glusterd2/*

The GD2 systemd unit now reads in environment variables from
/etc/sysconfig/glusterd2/*. This allows easier customization of GD2
installations, in particular customization of container images.

See `systemd.exec(5)` section 'Environment' for details about the format
of the environment files.

extras/systemd/glusterd2.service

@@ -6,6 +6,7 @@ Before=network-online.target
 Conflicts=glusterd.service
 
 [Service]
+EnvironmentFile=-/etc/sysconfig/glusterd2/*
 ExecStart=/usr/sbin/glusterd2 --config=/etc/glusterd2/glusterd2.toml
 KillMode=process