Calls to cfggen take considerable time. With batch mode, we will have the ability
to reduce number of calls from services.

Example of the batch mode command:
sonic-cfggen -t template-1.j2 -t template-2.j2,config-db -t template-3.j2,config-db -t template-4.j2,file1 -t template-5.j2,file2 --write-to-db.

template-1.j2 will be rendered to stdout since it is missing the dest part. stdout is default
config-db is a special keyword that will inject the rendered template into internal data structure. The internal data structure gets written to redis-db with --write-to-db switch. In the case the user would like to write to a file named config-db, it could be given as /config-db or ./config-db

signed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>