Added abstract helper interface for all storage backends (#135)

* Added abstract helper interface and implemented it for all storage backends

* Moved storage client initializations also to helper classes

* Fixed ssh init issue

* Moved script parameter to helper struct to simplify script init.

* Created sub modules. Enhanced abstract implementation.

* Fixed config issue

* Fixed declaration issues. Added config to interface.

* Added StorageProviders to unify all backends.

* Cleanup, optimizations, comments.

* Applied discussed changes. See description.

Moved modules to internal packages.
Replaced StoragePool with slice.
Moved conditional for init of storage backends back to script.

* Fix docker build issue

* Fixed accidentally removed local copy condition.

* Delete .gitignore

* Renaming/changes according to review

Renamed Init functions and interface.
Replaced config object with specific config values.
Init func returns interface instead of struct.
Removed custom import names where possible.

* Fixed auto-complete error.

* Combined copy instructions into one layer.

* Added logging func for storages.

* Introduced logging func for errors too.

* Missed an error message

* Moved config back to main. Optimized prune stats handling.

* Move stats back to main package

* Code doc stuff

* Apply changes from #136

* Replace name field with function.

* Changed receiver names from stg to b.

* Renamed LogFuncDef to Log

* Removed redundant package name.

* Renamed storagePool to storages.

* Simplified creation of new storage backend.

* Added initialization for storage stats map.

* Invert .dockerignore patterns.

* Fix package typo

cmd/backup/stats.go

@@ -30,14 +30,6 @@ type StorageStats struct {
 	PruneErrors uint
 }
 
-// StoragesStats stats about each possible archival location (Local, WebDAV, SSH, S3)
-type StoragesStats struct {
-	Local  StorageStats
-	WebDAV StorageStats
-	SSH    StorageStats
-	S3     StorageStats
-}
-
 // Stats global stats regarding script execution
 type Stats struct {
 	StartTime  time.Time
@@ -47,5 +39,5 @@ type Stats struct {
 	LogOutput  *bytes.Buffer
 	Containers ContainersStats
 	BackupFile BackupFileStats
-	Storages   StoragesStats
+	Storages   map[string]StorageStats
 }