Support for custom IHostService creation in HostBuilder

[mariotoffia]

Add methods:

• FromUri that uses a DockerUri to create a IHostService. This uri is arbitary. It also support other properties (see below).

   public HostBuilder FromUri(
      DockerUri uri,
      string name = null,
      bool isNative = true,
      bool stopWhenDisposed = false,
      bool isWindowsHost = false,
      string certificatePath = null) {/*...*/}

It will use "sensible" defaults on all parameters. Most of the case the uri is sufficient. For example if not providing the certificatePath it will try to get it from the environment DOCKER_CERT_PATH. If not found in the environment, it will default to none.

• UseHost that takes a instantiated IHostService implementation.

this closes isse #185