Add conga_maven_vault_password_file configuration variable with sensi…

…ble default (#7)

* Add a conga_maven_vault_password_file variable which defaults to the file configured for Ansible itself. This allows using the various Ansible configuration facilities for setting the password file (e.g. ansible.cfg) instead of being forced to set the ANSIBLE_VAULT_PASSWORD_FILE environment variable.
* Only include extra Maven command line arguments if they have a non-empty value to allow omitting them by setting them to empty.
* Bump the required Ansible version to 2.5 due to config lookup plugin usage

Add Debian stretch to platforms

Don't clean by default when building the configuration, as it will re…

…move all downloaded packages. 'clean' can be added to conga_maven_opts if required.