FilesystemImagePersistenceStrategy: exrtract method.

Addressed to #132

No functional changes.
FilesystemImagePersistenceStrategy: mark method as static.

Should be in 142bad09ff0517a83b9b1a6c86cecadbe4a3bf8b commit.

Addressed to #132

No functional changes.
TestStrategiesConfig: remove unused import.

Should be in 01f2442d455af204a5902307d67114387255d41e commit.

Reported by CheckStyle & PMD.

No functional changes.
FilesystemImagePersistenceStrategy: implemented get() method.

Addressed to #132