Upgrade etcd client to v3

[thomasferrandiz]

Description

This PR upgrades the etcd client to v3 to solve a CVE issue in the v2 client.

The registry tests now use go.etcd.io/etcd/tests/v3/integration instead of mock_etcd.
This is why there are many new dependencies.

Todos

• Tests: connection to a cluster using TLS
• Documentation: should I update Documentation/upgrade.md?
• Release note

Release Note

The v2 and v3 API of etcd are not compatible so if a live upgrade is performed, 
the flannel data stored in etcd must be migrated to be accessible by the v2 client after the upgrade of flannel.  

See the etcd documentation (https://etcd.io/docs/v3.5/op-guide/v2-migration/).

[luthermonson]

@thomasferrandiz can you merge all the code commits into one commit e.g. squash the lint changes into the rest of the code. I also would like you to split out the go.mod, go.sum and ./vendor changes into a separate commit with commit message of go mod or something so the code changes are completely split off from the go modules changes.

[thomasferrandiz]

@thomasferrandiz can you merge all the code commits into one commit e.g. squash the lint changes into the rest of the code. I also would like you to split out the go.mod, go.sum and ./vendor changes into a separate commit with commit message of go mod or something so the code changes are completely split off from the go modules changes.

Done in 6b6ba94 and 3a4cdf4.

[luthermonson]

@thomasferrandiz merged your netlink upgrade, can you rebase this and we'll get it merged

[thomasferrandiz]

@luthermonson the PR is rebased.

[luthermonson]

@manuelbuil give this a quick review but im feeling like we're done here now that ci works

[manuelbuil]

If we replace etcdv2 with etcdv3, we should also change the name of the subnet/etcdv2 directory, right?

[luthermonson]

@manuelbuil thought about this a bit too after we talked... maybe just rename to etcd? do we really need the version in the dir name?

[manuelbuil]

@manuelbuil thought about this a bit too after we talked... maybe just rename to etcd? do we really need the version in the dir name?

good point. I don't think we do. So we could change it to etcd

[thomasferrandiz]

I thought about changing the name but it would have made the diff hard to read so I decided not to rename the folder.
I'll do it now

[manuelbuil]

@sjoerdsimons @zhangzhangzf I think you are users of etcd subnet. Anything against this PR? If not we will merge it

[luthermonson]

@thomasferrandiz squash that last commit into the first and let's get this merged