Starts trove-guest if /etc/trove exists

Reasons:
- When provisioning a guest using heat code-path, the guest agent shows
  errors in first-boot.d.log, which is because first-boot tries to start
  trove-guest service before /etc/guest_info file is created by heat
  provisioning.

Changes:
- Adds condition to start trove-guest, to check existence of
  /etc/guest_info, if file exists, then start trove-guest.

Change-Id: I112822fa69b2d4a49d29457fa4b1acf06206c52e
Closes-Bug: #1278733
Merge "Starts trove-guest if /etc/trove exists"