This behavior is useless when aliases file is shared by multiple hosts with network drive.
Fixed by creating lock at the same place as aliases file.

[bug] Previously, the third parameter fed to external program was the value of ``host`` parameter of the list, not mail domain: If ``host`` parameter was not the same as domain name, external program could fail.
Fixed by feeding mail domain instead of ``host`` value. And added [% list.host %] template variable to use ``host`` value in template.
Note that ``host`` parameter may be deprecated in the future (See issue #43).

- If value of alias_manager parameter has full path of alias_manager.pl as value, Sympa::Aliases::Template module will be used instead of invoking alias_manager.pl.
- alias_manager.pl would be removed in the future.