Fix Gerrit welcome message format

* Fix Gerrit welcome message format. Everything other than the
  first line requires an extra space at the beginning of the line
  to preserve formatting.
* Change message-file option in gerrit_patchset-created.erb to be
  consistent with the other options (use equal sign)

Change-Id: I7d9ca046aca5ee906077e4eaf40797cd7f95f41f
Closes-Bug: 1348934