fix(coreos-base/coretest): Remove/replace coretest TlsDate checks

pull in https://github.com/coreos/coretest/pull/36 to fix TlsDate
checks and replace them with calls to ntpdate
Merge pull request #650 from brianredbeard/tlsdate-fix

pull in fix for coreos/coretest#35 to fix TlsDate checks