Add tls client options to api/cli

[kyhavlov]

This adds TLS client options to the api and cli for providing client certs and setting the TLS server name when connecting to Consul.

[slackpad]

Just one fix to the help text, looks good!

[slackpad]

"when `verify_incoming` is enabled"

[kyhavlov]

shouldn't the "is enabled." just be removed here? the CA settings can be used even when verify_incoming isn't enabled

[slackpad]

that's true

[arvenil]

So, with below I can pass my own HttpClient, with my own RoundTripper

if config.HttpClient == nil {
    config.HttpClient = defConfig.HttpClient
}

but the line

config.HttpClient.Transport.(*http.Transport).TLSClientConfig = tlsClientConfig

brakes it as it assumes standard RoundTripper implementation

[arvenil]

One way to solve that is to add new func NewHttpClient that will return ready to use consul http client - which is logical as this is what consul tries to do, provide his own HttpClient. Then user can use/modify it and pass to NewClient() via config. Now NewClient would either use provided HttpClient or if nil then run NewHttpClient() to create default one. This means I could always provide 1) my own client 2) my own client based on default consul client 3) default consul client when I don't provide any; and consul will never panic as it will never try to modify provided client (which he also shouldn't do as it might be default HttpClient).

I might be missing something with my train of thoughts so let me know if that's the case ;)