Overview checks

Basic and most useful checks for CB cluster
- RAM Overview
  - In use
  - unused
  - total in cluster
  - total allocated
- Disk Overview
  - in use
  - other data
  - free
  - usable free space
  - total cluster storage
- Buckets
  - active bucket count
  - ops/sec
  - disk fetch/sec
- Servers
  - active servers
  - servers failed over
  - servers down
  - servers pending rebalance
