config: check undefined config item

[Connor1996]

What problem does this PR solve?

We may miswrite the config name, and it is hard to find unless we check the config log carefully.

What is changed and how it works?

When PD restart, check the config undecoded field to find whether there is any undefined config.

Check List

• Unit test
• Manual test (add detailed scripts or steps below)

Test steps

• start PD with the default config file

>> ./bin/pd-server -config conf/config.toml
......
2018/12/06 21:38:16.482 leader.go:264: [info] PD cluster leader pd is ready to serve

• start PD with config file having undefined config

>> cat config1.toml
type = "pd"
>> ./bin/pd-server -config config1.toml
FATA[0000] parse cmd flags error: Config contains undefined item: type,

• start PD with config file having a wrong scheduler name

>> cat config2.toml
[[schedule.schedulers]]
type = "random-merge-schedulers"
args = [""]
>> ./bin/pd-server -config config2.toml
2018/12/06 21:50:04.637 log.go:274: [fatal] panic: can not create scheduler random-merge-schedulers: create func of random-merge-schedulers is not registered,

[nolouch]

/run-unit-test

[Connor1996]

PTAL @nolouch @disksing

[rleungx]

CI failed.

[Connor1996]

/run-unit-test