FIX: `cvmfs_server import` Parameter List in Usage Print

When using `cvmfs_server import` the key location parameter is optional, given the keys are already in the right location. Say you can do like this to resurrect a repository with it's standard configurations:
```
rm -fR /var/spool/cvmfs/test.local
rm -fR /etc/cvmfs/repositories.d/test.local
cvmfs_server import -o cvmfs_user test.local
```