bacula-director-mysql fails to install with reference to database '${db_name}'

[Nish Aravamudan]

[Impact]

 * During installation of bacula-director-mysql in 16.10:

mysql said: mysql: [Warning] mysql: Empty value for 'port' specified.
Will throw an error in future versions ERROR 1044 (42000) at line 2:
Access denied for user 'bacula'@'localhost' to database '${db_name}' .

This needs a backport of an upstream fix (http://www.bacula.org/git/cgit.cgi/bacula/commit/?h=Branch-7.4&id=074419ac0c9dbbde2e4d2f5ccb6d4ca85c6ec8a9)

[Test Case]

 * Install mysql-server and then install bacula-director-mysql. The latter will fail to configure with the above message from dbconfig, referring to ${db_name}.

[Regression Potential]

 * This is a backport of an upstream fix, which is something that has been done in the past upstream/in the Debian package. The regression potential is low, as it's a bugfix and the current version of the package does not work.

[Nish Aravamudan (nacc)]

Apologies, 16.04 doesn't need this fix, feel free to remove the nomination.

[Nish Aravamudan (nacc)]

Please test the potential fix version at:

https://launchpad.net/~nacc/+archive/ubuntu/bacula

Which results in 7.4.1~dfsg-1ubuntu1~ppa1 on yakkety or 7.4.1~dfsg-1.1~ppa8.is.actually.7.0.5+dfsg-4.1~ppa2 on xenial.