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/hooks.go

@@ -6,6 +6,8 @@ package main
 import (
 	"fmt"
 	"sort"
+
+	"github.com/offen/docker-volume-backup/internal/utilities"
 )
 
 // hook contains a queued action that can be trigger them when the script
@@ -50,7 +52,7 @@ func (s *script) runHooks(err error) error {
 		}
 	}
 	if len(actionErrors) != 0 {
-		return join(actionErrors...)
+		return utilities.Join(actionErrors...)
 	}
 	return nil
 }