build: bug fixes and slight improvements for flaky test script
build: bug fixes and slight improvements for flaky test script

* Fix the SECONDS_PER_DAY constant
* Properly convert the build timestamps into seconds before comparison
* Allow NUM_PREVIOUS_DAYS to be configured on the command line

Change-Id: I78c103baac7f2cffcac3c9f4fb7606f2f38f90e9
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3098
Tested-by: jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

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