Allow checks to have a minimum collection interval

To make sure they don’t run to often.

This is NOT a fully customizable check interval for a check.
It just allows to specify a minimum interval of time between 2
iterations of a check/instance.

See #797
init config should never be None
Merge pull request #1046 from DataDog/custom_check_period

Allow checks to have a minimum collection interval
Only send the ntp offset metric if we have it
Merge branch 'master' into Fix_bad_curl_behavior
Do not set options by default