Handle signout_redirect_url for non-SSO

Currently we default the signout URL to login.dso.mil in the package - https://repo1.dso.mil/platform-one/big-bang/apps/core/monitoring/-/blob/37b97faed79dfe5d9fc34a951c933ae0d636023c/chart/values.yaml#L827

In BB we override that value with the customer's SSO values - https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/1b0e36ee7664310fae68756abca33a5595079894/chart/templates/monitoring/values.yaml#L277

We should switch this in both cases to just default to the Grafana root URL (i.e. grafana.bigbang.dev) or potentially not override it (would have to test if this works). This will ensure:

• Logout works for both SSO and non-SSO
• Logout only logs out of Grafana, doesn't log out the Keycloak user too