Split base snapshots_client into v1 and v2

This patch splits the base client into v1 and v2 following the model
from the patch at [1] and also migrates those clients to tempest.lib.

[1] https://review.openstack.org/#/c/359442/

Change-Id: I03c254210aeff80173ced68cfa3f3e36ede10d22
diff --git a/releasenotes/notes/add-volume-clients-as-a-library-d05b6bc35e66c6ef.yaml b/releasenotes/notes/add-volume-clients-as-a-library-d05b6bc35e66c6ef.yaml
new file mode 100644
index 0000000..8125ab9
--- /dev/null
+++ b/releasenotes/notes/add-volume-clients-as-a-library-d05b6bc35e66c6ef.yaml
@@ -0,0 +1,12 @@
+---
+features:
+  - |
+    Define volume service clients as libraries.
+    The following volume service clients are defined as library interface,
+    so the other projects can use these modules as stable libraries without
+    any maintenance changes.
+
+    * encryption_types_client (v1)
+    * snapshots_client (v1)
+    * snapshots_client (v2)
+