ft: ZENKO-713: add callback to initManagement
ft: ZENKO-713: add callback to initManagement

The future management agent process will init the management before to
launch a websocket server. This commit adds an optional callback to the
initManagement function, call once the init is done, with the loaded
overlay as parameter.

scality/S3,scality/S3,scality/S3,scality/S3,scality/S3,scality/S3,scality/S3,scality/S3