Add multiple system log monitor support

[Random-Liu]

For #44.
Based on #81, #88 and #92.

Only the last commit is new.

This PR change the --log-monitor flag to --log-monitors. Multiple log monitor config separated by comma can be passed through --log-monitors, NPD will start a separate log monitor for each of them.

This makes it possible to use NPD to monitor different system log at the same time.

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

---

This change is 

[Random-Liu]

@dchen1107 Rebased. PTAL.

[dchen1107]

I only reviewed one commit (Add multiple log monitoring support) in this pr since all other should be covered / merged through different pr.

Two comments I have:

1. can we have unittest for parsing --system-log-monitors? at least make sure the corresponding system-log-monitoring goroutine is invoked, and there is no leakage, etc.
2. can we have a node / cluster e2e tests covering the new feature too?

[Random-Liu]

can we have unittest for parsing --system-log-monitors? at least make sure the corresponding system-log-monitoring goroutine is invoked, and there is no leakage, etc.

Will do.

can we have a node / cluster e2e tests covering the new feature too?

I manually tested this, and this works. I'll add a test case in NPD e2e test for this feature.

[Random-Liu]

@dchen1107 Add a new commit to test goroutine leak.

In fact, with the unit test, we are still not sure whether a failed log monitor will leak any other resources instead of goroutine. However, resource leaking is originally hard to capture in unit test.

[Random-Liu]

@dchen1107 The PR is ready for another review. PTAL!

[dchen1107]

LGTM