Added support for overriding the package name for bacula::filedaemon

Different major versions of Bacula are not compatible and operating systems may
provide more than one bacula package/port. Added a new parameter, $package_name,
that allows overriding the default package name defined in bacula::params. While
this approach to the package versioning mixes up policies and OS abstraction a
bit, code-vise it's much cleaner than the alternatives.

This change also moves away from using "include bacula::params" to inheriting
bacula::params in bacula::filedaemon. This is done to ensure that the contents
of the bacula::params class can be used as default values for
bacula::filedaemon.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>