Use nearest zero-based pool name for expansion

[allanrogerr]

Issue summary

After tenant expansion and decommission, further expansion is not always possible using the MinIO tenant console.

As an example, in a deployment with pool-0 and pool-1, after pool-0 decommission, a subsequent expansion by the console tries and fails to create pool-1 which already exists.

This is because the MinIO tenant console automatically names pools based on the number of pools in existence. If there is 1 pool, then the logic will try to create pool-1 - which may be incorrect as shown above. This logic needs to consider examining the pools already in existence before determining which pool name to use.

Test summary

Reproduce issue on master
Test 0 - As reported by user, Create using Console, Expand using Console, Decommission pool-0, Expand tenant using Console

Fix on improve-poolname-search branch
Test 1 - Create tenant using console, Expand using kubectl-minio, Decommission pool-0, Expand tenant using Console
Test 2 - Create tenant using kubectl-minio, Expand using console, Decommission 0pool, Expand tenant using Console
Test 3 - As reported by user, Create tenant using Console, Expand tenant using Console, Decommission pool-0, Expand tenant using Console
Test 4 - Create tenant using kubectl, Expand tenant using kubectl, Decommission pool-1, Expand tenant using Console

Verbose tests here:
https://github.com/allanrogerr/public/wiki/operator%E2%80%901785

Fixes: #1785

[shtripat]

lgtm