Factor up (most) CONF value in clients.Manager

Factor up CONF dependencies from the client manager class, so
that the part of it independent from CONF and unstable interfaces
can be moved to the manager base class, and be useful to external
consumers (such as plugins).

An helper function is defined in config.py to help extract from
configuration the parameters relevant for service client __init__.
This is used for Tempest own service clients and will be used by
plugins as well.

Partially-implements: bp client-manager-refactor

Change-Id: I19cdcf6f84144f6b1b0223a41a52f5b6e4b8fdd1
diff --git a/releasenotes/notes/service_client_config-8a1d7b4de769c633.yaml b/releasenotes/notes/service_client_config-8a1d7b4de769c633.yaml
new file mode 100644
index 0000000..3e43f9a
--- /dev/null
+++ b/releasenotes/notes/service_client_config-8a1d7b4de769c633.yaml
@@ -0,0 +1,6 @@
+---
+features:
+  - A new helper method `service_client_config` has been added
+    to the stable module config.py that returns extracts from
+    configuration into a dictionary the configuration settings
+    relevant for the initisialisation of a service client.