Support specifying repo config via env (#363)

This allows the user to specify repo configuration via environment variables when a config file is not available.

This is a temporary fix - ideally we would leverage Viper for this but we currently only use Viper if a config file
is available so that needs to be refactored a bit.

[1]
`TENANT_ID` (already supported)
`BUCKET`
`ENDPOINT`
`PREFIX`

src/cli/config/storage.go

@@ -1,6 +1,8 @@
 package config
 
 import (
+	"os"
+
 	"github.com/alcionai/corso/cli/utils"
 	"github.com/alcionai/corso/pkg/credentials"
 	"github.com/alcionai/corso/pkg/storage"
@@ -59,9 +61,9 @@ func configureStorage(vpr *viper.Viper, readConfigFromViper bool, overrides map[
 
 	s3Cfg = storage.S3Config{
 		AWS:      aws,
-		Bucket:   first(overrides[storage.Bucket], s3Cfg.Bucket),
-		Endpoint: first(overrides[storage.Endpoint], s3Cfg.Endpoint),
-		Prefix:   first(overrides[storage.Prefix], s3Cfg.Prefix),
+		Bucket:   first(overrides[storage.Bucket], s3Cfg.Bucket, os.Getenv(storage.BucketKey)),
+		Endpoint: first(overrides[storage.Endpoint], s3Cfg.Endpoint, os.Getenv(storage.EndpointKey)),
+		Prefix:   first(overrides[storage.Prefix], s3Cfg.Prefix, os.Getenv(storage.PrefixKey)),
 	}
 
 	// compose the common config and credentials

src/pkg/storage/storage.go

@@ -20,6 +20,14 @@ var (
 	errMissingRequired = errors.New("missing required storage configuration")
 )
 
+// envvar consts
+// TODO: Remove these and leverage Viper AutomaticEnv() instead
+const (
+	BucketKey   = "BUCKET"
+	EndpointKey = "ENDPOINT"
+	PrefixKey   = "PREFIX"
+)
+
 // Storage defines a storage provider, along with any configuration
 // requried to set up or communicate with that provider.
 type Storage struct {