Logging: Enhance logging with separate files and timestamps

Logging: Enhance logging with separate files and timestamps

This patch adds following:
1. overall.log file is change to vsperf-overall-timestamp.log files
2. The overall.log files will include timestamps too.
3. Separate log files for infrastructure metrics.
4. All timestamps of the logfiles for a given test will be same.
5. Store the timestamp in 'LOG_TIMESTAMP'
6. Create metrics-log only if 'COLLECTOR' is set to 'Collectd'

JIRA: VSPERF-586

Change-Id: I35d373a66b1074f6ee1a440ffc82bc3db03f36e7
Signed-off-by: Sridhar K. N. Rao <sridhar.rao@spirent.com>