Migrate etcd to use 'etcd-io/etcd' and update relevant paths

[davissp14]

Given the coreos/etcd client was deprecated back in 2015, let's work to move to etcd-io/etcd.

[davissp14]

make tests in Agola are failing with:

panic: codecgen version mismatch: current: 8, need 10. Re-generate file: /go/pkg/mod/github.com/coreos/[email protected]+incompatible/client/keys.generated.go
goroutine 1 [running]:

Not quite sure what to do about that. Tests work fine locally.

[sgotti]

@davissp14 Thanks for the PR. Sorry but I don't understand the title and why this is needed. We're already using for etcdv3 go.etcd.io/etcd client v3, just the 3.4 version with a workaround since only from the recently released etcd v3.5 they fixed the wrong module path (missing /v3).
github.com/coreos/etcd is used for etcdv2 and required by libkv.

[davissp14]

@sgotti My mistake, the dependencies here are a little confusing... For context, I have a fork where i'm introducing the ability to specify Store specific credentials for Etcd. I was not able to get authentication to work properly using the current client version. Bumping to 3.5 seems to have addressed my issues.