Dex configuration docs are incomplete

[stoehdoi]

Is this a BUG REPORT

What happened:
I followed the instructions to use Dex for litmus chaos center but it did not work when following the documentation. Apparently, the litmus-server deployment requires some more environmental variables to be set which are not described in the how-to. Those are:

    DEX_OAUTH_CLIENT_SECRET: <client secret for dex static client>
    DEX_OAUTH_CLIENT_ID: <client ID of dex static client>

The two ENV vars are required in the oauth2 config:
https://github.com/litmuschaos/litmus/blob/2.6.0/litmus-portal/authentication/api/handlers/rest/dex_auth_handler.go#L26-L27

What you expected to happen:
The litmus docs to include all the required details to make litmus <-> dex integration work

How to reproduce it (as minimally and precisely as possible):
Following the documentation will lead to an error page when trying to authenticate via the /auth/dex/login endpoint complaining about the client_id to be empty ("").

Anything else we need to know?:
There had been some more bugs that seem to be fixed in master already like the change of ENV var names (DEX_SERVER -> DEX_ENABLED, etc.)