When PING_NUMBER is nonzero, there are many nodes that are immediately marked as unhealthy

**Describe the bug**
If I start off with 50 healthy nodes and then set PING_NUMBER to 20, I notice that roughly 30 nodes get marked as unhealthy. It appears that 20 nodes ping 20 nodes instead of 50 (all) nodes pinging 20 nodes. When I unset PING_NUMBER, the nodes go back to being healthy.

I think the bug is [here](https://github.com/bloomberg/goldpinger/blob/ca676bcbc2da02fdd5b796689ea727896ad15c74/pkg/goldpinger/client.go#L54). It seems to me that `AllPods()` should be "checked" but only `SelectPods()` are actually being checked which results in `SelectPods()` pinging `SelectPods()`.

**Expected behavior**
All nodes ping PING_NUMBER nodes and no nodes are marked as unhealthy as a result of applying change to set PING_NUMBER.

Start in pkg/goldpinger/client.go at the referenced line and compare how AllPods() and SelectPods() are used when PING_NUMBER is 20. Reproduce with 50 healthy nodes, then verify that all nodes ping the selected nodes and remain healthy after the setting changes.

Written by the indexing model from the issue text.