fix: add missing url in grafana datasource

[takirala]

What does this PR change?

grafana-datasource configmap is being created with invalid datasource as the url is empty. I think this was an overlook when thanos support was removed (in 279d7be#diff-406c7314a1029b6fe683e8a2c4bf1f7f52222794f542eb30a5a0aec78e022e28). Please let me know if I am missing some context.

Does this PR rely on any other PRs?

How does this PR impact users? (This is the kind of thing that goes in release notes!)

• Fixes the url in grafana datasource configmap when using an external grafana using the sidecar.

Links to Issues or tickets this PR addresses or fixes

What risks are associated with merging this PR? What is required to fully test this PR?

• Create a kubecost cluster by disabling in-built grafana and configure an external grafana. The datasource will be created with an invalid value. Adding the fix in this PR will make the datasource work correctly.

How was this PR tested?

I updated the configmap for grafana-datasource and was able to browse grafana correctly.

Have you made an update to documentation? If so, please provide the corresponding PR.

[takirala]

Ping @jessegoodier @ivankube @cliffcolvin PTAL 🙏
I am not sure i pinged the right people, please redirect as needed, thank you!