Add a method to generate password with
1: use of both upper- and lower-case letters (case sensitivity)
2: inclusion of one or more numerical digits
3: inclusion of special characters, e.g. @, #, $ etc.
4: at least 15 characters in length
Closes-Bug: 1448217
Change-Id: I86bf157f1bdb44f5fc579dc5317784fe31df8521