Add a time constraint while evaluating regressions in performance reports.

For very short queries, using percentage as a strict bound for detecting regression does
not cut it. This introduces a flag through which the user can set a value (in seconds),
which will only attempt to detect a regression if the time difference is equal to or
greater than the value.

Change-Id: I95bde65afb8fd564666223fe069537e2483ec98e
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1498
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins