Add sardana_init_hook
Created by: catunlock
Add sardana_init_hook to SardanaDevice for allowing the subscribe to pool's elements attribute in the init of the MacroServer.
Based on the response of Michal to tango-controls/pytango!419 (closed). Modify the run_tango_server to replicate the behavior of post_init_callback found in PyTango High-level API which is invoked before the server enters the event loop. Fixes #674 (closed).