Add support for etcd client version 3

[khvysofq]

Hi, kpacha. This PR is primarily to support the functionality of the ETCD clientv3. The major changes are as follows

• Add clientv3.go、clientv3_testing.go in the root of the source code.
• Move some common parts into the config.go

Both modes are supported through profile configuration, just add "client_version": "v3" option in extra_config

"extra_config": {
    "github.com/devopsfaith/krakend-etcd": {
        "machines": [ "http://192.168.110.111:2379" ],
		"client_version": "v3",
        "options": {
            "dial_timeout": "5s",
            "dial_keepalive": "30s",
            "header_timeout": "1s"
        }
    }
}

• "client_version": "v3" using the etcd clientv3
• "client_version": "v2" using the etcd clientv2 (old version)

The default value is v2, if you are not set "client_version". I have tested in my local machines.

---

Among other things, there is one small problem. I used go mod to manage my go packages. but in the original code of the example/main.go, the github.com/devopsfaith/krakend-etcd(v 3.3.10 or newer version) and github.com/devopsfaith/krakend-viper conflict when load package github.com/ugorji/go

• github.com/devopsfaith/krakend-etcd(v 3.3.12) need github.com/ugorji/[email protected]
• github.com/devopsfaith/krakend-viper auto need github.com/ugorji/go/codec v0.0.0-20181204163529-d75b2dcb6bc8

They need the same package, but not the same directory and not the same version. The go mod can't handle this situation. see spf13/viper#658

So, in the example/main.go, I can only change github.com/devopsfaith/krakend-viper to github.com/devopsfaith/krakend/config

[kpacha]

This is a great contribution!

Please, check my comments

[khvysofq]

HI, thanks for you PR review, I have solved, See the commits.

[kpacha]

@khvysofq I've added two more comments regarding two minor details and I think it's almost ready to be merged.

Again, thanks for the contribution to the project

[khvysofq]

The KrakenD is a very promising project . I am honored to contribute to this project. Anyway if you have any suggestions for this PR, please point it out. I want do it better as krakenD project do.