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.
Shouldn't the master key be imported, too, if it is present?