Count memory and disk slots once in cells state manager

If multiple instance types in a cell had the same memory_mb value or
root_gb + ephemeral_gb added to the same value, then capacity counts
would be inaccurate since slots would get counted multiple times.
Rather than counting based on instance type it is now based on the set
of memory or disk values pulled from instance types.

Change-Id: Iaded7fa69dadb4aa0335ef1569975bdfcb2767c6
Closes-bug: #1276728
Merge "Count memory and disk slots once in cells state manager"