Move WsAgent and Terminal Checkers to agents module, check and clean

[gazarenkov]

1/
org.eclipse.che.plugin.docker.machine.ext.DockerMachineExtServerChecker.class
and
org.eclipse.che.plugin.docker.machine.ext.DockerMachineTerminalChecker.class
are plased to docker machine plugin but they have nothing to do with docker and machines.
Let's move them to agents module

2/
It seems this will not correctly work for the all configs (only when we have agent as a local archive ect). It has to be validated. Also we need to consider AgentRegistry API to check this kind of things, let's discuss then