Add arbitray system log support

[Random-Liu]

For #44.

Based on #81.

This PR:

• Add PluginConfig for plugin specific configuration.
• Add source in journald plugin configuration to make sdjournal SYSTEM_IDENTIFIER configurable.
• Add timestamp regular expression, message regular expression and timestampFormat configuration in syslog plugin configuration. (We plan to change this to be filelog, because it can handle not only syslog)

TODOs for this work:

• Add multiple configuration files support. NPD should start a system log monitor for each passed in configurtion, so that it can monitor different services at the same time.
• Rename syslog to filelog.
• Rename kernel monitor to system log monitor, change all "kernel" reference in the code.
• Optimize the efficiency to make the system log monitor more light weight.
• Add document for the new behavior.
• Update kubernetes e2e test to test the new behavior.

/cc @dchen1107 @kubernetes/node-problem-detector-reviewers

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This change is 

[Random-Liu]

I've updated the e2e test locally according the change in this PR, and the PR passed the test.

Will update the e2e test in Kubernetes repo after this PR is merged.

[dchen1107]

Can you rebased this one? #81 was merged, and I am reviewing this one today.

[Random-Liu]

@dchen1107 Rebased. :)

[dchen1107]

LGTM