Removes a dependency on RDO for install etcd in CentOS images.
Aligns version of etcd across all distributions, see below:
```
(venv-kolla) [will@juno kolla]$ docker run --rm -it kolla/ubuntu-source-etcd:yoga bash
()[etcd@730a011b670a /]$ etcd --version
etcd Version: 3.2.26
Git SHA: Not provided (use ./build instead of go build)
Go Version: go1.13.7
Go OS/Arch: linux/amd64
```
```
(venv-kolla) [will@juno kolla]$ sudo docker run -it kolla/centos-source-etcd:yoga bash
()[etcd@9aa486c9e94f /]$ etcd --version
etcd Version: 3.2.21
Git SHA: 3ac81f3
Go Version: go1.12.8
Go OS/Arch: linux/amd64
```
```
(venv-kolla) [will@juno kolla]$ docker run --rm -it kolla/debian-source-etcd:yoga bash
()[etcd@b49cb5fcdf87 /]$ etcd --version
etcd Version: 3.3.25
Git SHA: Not provided (use ./build instead of go build)
Go Version: go1.15.9
Go OS/Arch: linux/amd64
```
This will also allow us to update to a more recent version in the
future. We have to be careful not to jump by more than one point release
to ensure compatibility:
https://etcd.io/docs/v3.5/upgrades/upgrading-etcd/#upgrading-an-etcd-v3x-cluster
Change-Id: I62a34256fb7395d0448af169ac3cf036f60cd290
91d22dd40f