Rename master answer template, add enhancements.

In commit d9be734, the name of the built-in template was changed and no
longer conformed with how the provisioner found default answer files.
Default answer files are used in the absence of a user provided answer.
They contain safe defaults to bring up a generally default master or
agent.

This commit restores the master.txt.erb file as well as chaging a few
values to safer defaults. The previous file assumed a pre-installed
MySQL server, the new settings assume the PE installer will be
installing the MySQL server. The default username and password were
changed to vagrant@example.com and vagrant respectively. These new
default console credentials reflect the defaults that Vagrant boxes are
supposed to contain.