Refactor: factor out import_keychain() from import()
add `cvmfs_server mkfs -k` option for key import
allow import of keychain from given location
detect existing keys in /etc/cvmfs/keys
TEST: import existing keychain in mkfs
FIX: restrictive permissions are default
TEST: check permissions of imported key files
FIX: check more careful for existing keys
import masterkey if available in `cvmfs_server import`
Merge pull request #529 from reneme/feature-provide_keychain

Feature: Import Keychain when doing `cvmfs_server mkfs`

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:

    Provide an import location with `cvmfs_server mkfs -k`
    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.