Kubernetes-compatible combined 1+2 PMM Client Docker image

As a company, we need to have backward compatibility pmm1 and pmm2 docker client image so that we can simplify usage of our pmm-client container in K8s  

Successful resolution of the feature:

• The new docker container with pmm1 and pmm2 clients/agent inside and which will use needed client based on the version of the configured pmm server.  

• The variables we pass to the container must not be different for pmm1 or pmm2 

Expand with PMM 1.x compatibility. See https://github.com/Percona-Lab/pmm-submodules/pull/447/files
Use Python 3, not bash.