Migrate backups clients to tempest.lib

Split and migrate v1 and v2 backups__client to tempest.lib.

Partially implements blueprint consistent-service-method-names

Change-Id: I9d6ef9f069c449f392c245fdac10996517a1cc73
diff --git a/tempest/lib/exceptions.py b/tempest/lib/exceptions.py
index e3f25e6..a5c6b1b 100644
--- a/tempest/lib/exceptions.py
+++ b/tempest/lib/exceptions.py
@@ -239,3 +239,7 @@
 
 class PluginRegistrationException(TempestException):
     message = "Error registering plugin %(name)s: %(detailed_error)s"
+
+
+class VolumeBackupException(TempestException):
+    message = "Volume backup %(backup_id)s failed and is in ERROR status"