Set check_mozillian_username execution soft time limit.

check_mozillian_username has been taking more than 120secs on celery to
complete and thus hitting the execution soft time limit. Raise the time
limit only for this function to 5 mins to solve this.

Also restore the previous execution schedule (once per day) of the task.
Merge pull request #726 from glogiotatidis/task_time_limit

Set check_mozillian_username execution soft time limit.