Update Optimal Replica Placement Logic

[4n4nd]

Add logic to check for zones with most available replicas

Resolves #74

Current logic:

Let's assume we have 4 zones and we have a cluster with 3 primaries and 2 replication. So 9 pods will be scheduled across the cluster.

Step 1: Pods scheduled	Zones:	Z0	Z1	Z2	Z3
Scheduled pods:	2	2	2	3

Step 2: Primaries assigned	Zones:	Z0	Z1	Z2	Z3
Available pods:	1	1	1	3
Shard-0 pods:	1	0	0	0
Shard-1 pods:	0	1	0	0
Shard-2 pods:	0	0	1	0

Step 3: Replicas assigned for Shards-0/1	Zones:	Z0	Z1	Z2	Z3
Available pods:	0	0	0	2
Shard-0 pods:	1	1	1	0
Shard-1 pods:	1	1	0	1
Shard-2 pods:	0	0	1	0

We can see here the operator does not have any other option but to schedule both the pods for Shard-2 in Z3.

New logic:

Step 1: Pods scheduled	Zones:	Z0	Z1	Z2	Z3
Scheduled pods:	2	2	2	3

Step 2: Primaries assigned	Zones:	Z0	Z1	Z2	Z3
Available pods:	1	1	1	3
Shard-0 pods:	1	0	0	0
Shard-1 pods:	0	1	0	0
Shard-2 pods:	0	0	1	0

Step 3: Replica assignment Shard-0

Here assignment will start from the zone that has the highest availability (Z3)	Zones:	Z0	Z1	Z2	Z3
Available pods:	1	1	0	2
Shard-0 pods:	1	0	1	1
Shard-1 pods:	0	1	0	0
Shard-2 pods:	0	0	1	0

Step 3: Replica assignment Shard-1

Zones:	Z0	Z1	Z2	Z3
Available pods:	0	1	0	1
Shard-0 pods:	1	0	1	1
Shard-1 pods:	1	1	0	1
Shard-2 pods:	0	0	1	0

Step 4: Replica assignment Shard-2

Zones:	Z0	Z1	Z2	Z3
Available pods:	0	0	0	0
Shard-0 pods:	1	0	1	1
Shard-1 pods:	1	1	0	1
Shard-2 pods:	0	1	1	1

[cin]

Sorry for the delay in testing this @4n4nd. I'll try to carve out some time this afternoon to test.