fix(omaha_request): always send pings

In the ping_only requests always send it instead of limiting it to once a
day. CoreOS wants to have active pings from all servers hourly instead
of daily.
chore(tests): remove broken kernel image test
chore(omaha_request): remove all logic around pings

Drop all logic around pinging daily since we send a ping in every
request.
Merge pull request #32 from philips/ping-faster

fix(omaha_request): always send pings