Data sources refactoring

[farnasirim]

Already in progress.
TODO:
1- workaround for templating, previously done by "Repo"s with references to multiple data sources.
2- etcd_datasource is getting pretty big. Perhaps the implementations for different parts of the MasterDataSource interface should be separated out into multiple files.
3- Switching DHCP server's back-end data source from lease pool too an implementation of DHCPDataSource.