regression.py: fix a typo and cleanup

Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: round values of "%Diff between Avg"

Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: reserve '+' signal  for Positive values

Xigao found Positive "Significance" value is removed "+" signal,
it's introduced in [1]

[1] https://github.com/autotest/virt-test/pull/1516

Reported-by: Xiaomei Gao <xigao@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: convert test arguments back to integer

Xigao found Block_size/Iodepth/Threads is valued by float.

Signed-off-by: Amos Kong <akong@redhat.com>
regression.py: add test data to report

This patch added the job finished time to report.

Signed-off-by: Amos Kong <akong@redhat.com>
Merge pull request #1526 from kongove/perf-regression

fix 3 issues reported by xigao in regression.py