Management of an encryption keys is common.

Some key changes within this commit:
- Common part of storing encryption keys moved to
  swift.common.key_manager. Also, the package have interface function
  which named as 'get_driver' to initialize instances of drivers.
- Object server encryption driver have similar interface to get driver
  instances.
- Changed sample configuration files for object and proxy servers.
- Improved unittests for encryption implementation.

Change-Id: I82beb7d383ae2cd2f7d20a26e4adcb39c943ba1a
Improved method of loading drivers.

* Add function create_instance which support to import class by path,
  validate its subclass and create instance. Wrote unit tests.
* Replace mechanism of importing key manager and crypto drivers.
* Add descriptions to samples of configuration.
* Add dummy key manager driver.

Change-Id: I7370f4dd9c58b346667ee7baab851997ae0a7700
Improved pluggable mechanism of encryption.

* Cleaned interface of drivers. Added encryption context to exchange
  of security data between unified encryption/decryption methods.
  Instance of key manager created externally towards drivers.
* Renamed FakeDriver to DummyDriver.
* ObjectController and DiskFile used new interface of drivers.

Change-Id: I4acb253b9a0f4c348fc6f27326e4219d086efbef