Feature: Import Keychain when doing `cvmfs_server mkfs`

@jblomer: please review carefully.

This allows the import of an existing keychain when creating a new repository. The code for this was already part of `cvmfs_server import` in the context of importing a full repository backend storage. Thus I factored it out into a new helper function: `import_keychain()`.

There are two ways to import the keychain in `cvmfs_server mkfs`:

1. Provide an import location with `cvmfs_server mkfs -k`
2. Copy the keys directly into */etc/cvmfs/keys* and let `cvmfs_server mkfs` pick them up automatically

In both cases, one needs to name the key-files properly, like: *[repo_name].{pub,key,crt,masterkey}*. The script detects if importing keys from an external location would overwrite existing keys and complains.

Additionally there is an integration test case verifying the new feature.