Fix for coreos/coretest#35 (coretest should be using ntp...)

Previously coretest was using tlsdate to check and make sure that
it could perform outbound network connectivity to a tlsdate
compatible endpoint.  This change switches to ntpdate using similar
semantics.
Fixing NTP to comply with the Go style guide

Go style guide says that acronyms should be fully
capitalized: TestNTPDate
Merge pull request #36 from brianredbeard/tlsdate-fix

Fix for coreos/coretest#35 (coretest should be using ntp...)