Be able to change the logfile location for memcached.

I was unable to specify my own logfile name/location. Debian had a small inconsistency based on the docs:

`memcached['logfilename']` - logfile to which memcached output will be redirected in /var/log/$logfilename.

Although I removed the /var/log/$logfilename location to a full path.
This is not consistent with the way the other templates are set up. This should be:
```
logfile /var/log/<%= @logfilename %>
```
Either that, or all the other templates should be updated to support full paths.