Make syncPercentage configurable (#675)

* Make syncPercentage configurable

 - Use `syncbeaconfollower.syncPercentage` in config.json to specify
   the desired percentage of nodes (being followed) that we need to be
   synced compared to in order to consider the node synced.
 - syncPercentage has to be in [0.5, 1.0] interval.
 - Use default value of 0.5 in case of invalid config.

* Add syncPercentage to config.default.json

* Fix syncbeaconfollower configure order