add counter for groups that could be created considering BSC logic

[antonkovalenko]

Currently as a cluster admin, I want to know how many groups of every media kind I can create right now to monitor cluster capacity and act in advance.

There is AvailableGroupsToCreate sensor in storage_pool_stat service. As we discovered it doesn't reflect inactive pdisks. And reports number of groups to create in a situation when all pdisks are ready.

To avoid situations when too many pdisks are marked inactive and cluster capacity is decreased we need one more sensor that will report amount of groups to create from Blob Storage Controller point of view