Validation for all SD configs

[gouthamve]

There is no validation for some SD configurations which might lead to issues like: #3066 The KubernetesSD config is a good example where validation happens: https://github.com/prometheus/prometheus/blob/master/config/config.go#L1049-L1065

This should be done for every SD mechanism.

[krasi-georgiev]

DIBS

There is no pending PR for this right?

[serathius]

@krasi-georgiev Are you still working on it? if not can I take it over?

[krasi-georgiev]

@serathius , nope you can go ahead, I got lost in other PR's

[serathius]

@gouthamve I added validation based on prometheus documentation, it should be enough to eliminate case in #3066
Should we consider more aggressive version? Writing full rules for openstack optional flags looks impossible without breaking someone.

[simonpasquier]

@gouthamve what do you think of closing this issue? The scope being unclear, it is impossible to declare that it is done.

[palash25]

I see that the consul SDConfig doesn't have any other validations except for the Server field, are any other validations required for other fields such as Password or Token? cc @gouthamve