Stateless Ruler requires objectStorageConfig definition / Ruler fails to start

[chipzzz]

Stateless Ruler still requires objectStorageConfig to be defined, where we wouldn't utilize object store since we will be remote writing the rules.

Even when providing spoof values for the object store in order for template checks to pass, k8s then tries to create pvc and fails and thus the ruler fails to start

This arg needs to be optional and the env vars also optional/ checks to accommodate stateless ruler, when remoteWriteConfigFile is defined for Stateless Ruler

ref: #263