Add remote host binding for JenkinsRule

[fr123k]

The reason for this change is that the JenkinRule based tests were all running
inside a docker container and i needed a way to bind it a non localhost address
in order to access it from outside the running container.

The host binding for the JenkinsRule is configurable with the host
property. The default value is localhost this keeps the backward
compatibility.

The following code snippet shows how to bind the JenkinsRule instance

@Rule public JenkinsRule j = new JenkinsRule() {{
    host = null;
  }
};

to all network interfaces. This is similar to set the host value to 0.0.0.0.

The test case JenkinsRuleHostIPTest validates that the assignment to a specific
host ip address is working correctly.

The test case JenkinsRuleAllHostInterfaceTest validates that the assignment to all
network interfaces is working correctly.

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[oleg-nenashev]

Looks good to me 👍

[jglick]

This is basically #84 I think, and is potentially problematic for the same reason. See https://github.com/jenkinsci/kubernetes-plugin/blob/55e01eaca472ddaf7f4ae15ea9569571db6672ab/src/test/java/org/jvnet/hudson/test/JenkinsRuleNonLocalhost.java

[fr123k]

That's true if you bind JenkinsRule instance to 0.0.0.0. Then its possible to access it from remote depends on the network setup.

I could add a log warning too.

The default is still localhost and has to be conciously overwritten with a different IP or hostname.