html-escape warmup task's texts

Because it's always good idea to do.

Change-Id: Id3e457d6ac739c1fe2f8e70c1d539b2c42743cf7
Reviewed-on: http://review.couchbase.org/32889
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>