Move preseed variable definition to defaults

Default root password lookup for Debian preseed script has been moved to
'defaults/main.yml' and should be easier to change or switch to static
password in inventory if necessary.

Default username and SSH public key lookups have also been moved there,
and SSH key is now sourced from `ssh-agent` instead of directly from
default file.
Merge pull request #4 from drybjed/sshkey

Move preseed variable definition to defaults