Incorporating minor review comment

Adding option to specify duration for which cassandra log to be collected.

When fab attach_logs_cores collect cassandra logs, it collects from
system uptime. Now if the system is up for long time then fab attach_logs_cores
take very long time to execute as cassandra long collection takes time. Adding
option to specify the last duration for which cassandra logs to be collected.
Change-Id: I8c01b1cd808d8c600fbe699388058ac4aabcf075
Merge "Incorporating minor review comment" into R2.0