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: Ief6e4e877c43216b554e45d4be69c04de137c324
Merge "Incorporating minor review comment"