config: introduce initial support of vshard

This patch introduces initial support for the vshard configuration.
There is still a lot to be done in both vshard and the config to be able
to run vshard naturally. Key support restrictions introduced in the
patch:
1) at the moment there are only two roles: storage and router;
2) the entire config is considered a configuration for one sharded
system;
3) the rebalancer is currently disabled;
4) The router can automatically find all masters, but once all masters
are found, any changes to the masters will be ignored until
vshard.router.cfg() is called manually.

Closes #9007

NO_DOC=Will be described when full support for vshard is introduced.