Fixes #6822/bz1117554 - Tasks search breaks for defualt searches

Prior to this commit
Task.search_for("foo") would search with an invalid whereclause
Something like
"""....
OR foreman_tasks_locks."" ILIKE 'Red' OR users."" ILIKE 'Red' OR
users."" ILIKE 'Red')
"""

This commit fixes that by hard coding the column names to search on if
none are provided.