TLS verify is skipped by default

[simonswine]

Looking into the TLS implementation of cortex, we are not validating the server side certificates by default.

I think we should validate by default and expose a skip verify flag/config option to get the current behaviour if necessary.

The problem lies pretty much in this line:

cortex/pkg/util/tls/tls.go

Line 45 in a4aad5d

InsecureSkipVerify: true,