Consul should use prometheus/common HTTPClient

[roidelapluie]

With prometheus/common#308, consul should be able to use the full common's http client.

It means that it can get the ability to read its token and basic auth password from a file.

Currently we only use the TLS part.

Notes:

• We must stay compatible with the current config (when unmarshalling, turn provided username and password into http client basicauth).
• We should check that authorization is not set at the same time as consul token. If it is the case, error.
• if a consul token is passed, it should still be passed to the consul client, because I think it uses X-Consul-Token, which is needed to connect to old consul, that does not support Authorization header. So consul_token should still be documented.
• if username and basic_auth.username are set, we should error. Same for password.

[roidelapluie]

cc @LeviHarrison

[roidelapluie]

For the record, Levi is working on this.