Fix DistributesShardedDOMDataTree.ProxyProducer's getShardAccess

DistributesShardedDOMDataTree.ProxyProducer's getShardAccess works only
for subtrees that are rooted at some registered prefix based shard.
Moreover subtree has to be one of the subtrees specified in
DistributedShardedDOMDatatTree's createProducer method.

This is way more strict than what is required by CDSDataTreeProducer's
API. Pass ProxyProducer's implementation current shard layout, so
producer can lookup corresponding shard for specified subtree in
getShardAccess method. One-to-one mapping between shards and subtrees
is no longer required.

Change-Id: I765567d34c803a85b4be8a6e10fd81b6f64a1610
Signed-off-by: Jakub Morvay <jmorvay@cisco.com>