Allow --default-password with --gen-answer-file

Previously, it was not possible to specify a default password when
creating an answer file. This patch fixes that by allowing to run
"packstack --gen-answer-file=<file> --default-password=<password".
The answer file will have the specified default password included.

Change-Id: I52b55f02468f79ebbab4793e727810ed99eaac69
Merge "Allow --default-password with --gen-answer-file"