This patch enables the HAProxy webstats for all the configures
backends and frontends.
A password entry is added to user_secrets.yml for the webstats
password.
It also adds variables for port number, username and password
which can be overridden in user_variables.yml appropriately.
Change-Id: Iec866ad124bec6fb0b8524a966adf64e22422035
Closes-Bug: #1446432