Enhancing the make_test_data script

-Adding random boundaries rather than exact value for samples
-Allowing to pass datetime object or string
-Extracting make_test_data func, so it can be imported elsewhere
-Converting the integer time span to datetime

Change-Id: Iffbc8d9711b2f301a1f28b49980691ab34d17c20
Related-Bug: #1349791
Merge "Enhancing the make_test_data script"