Add a new credential provider to use a list of creds

This commit adds a new credential provider to use credentials provided
from a list of available credentials.

Partially implements bp test-accounts
Change-Id: I69c3105f8b413102ea567ccb92965572ce03d679
diff --git a/etc/accounts.yaml.sample b/etc/accounts.yaml.sample
new file mode 100644
index 0000000..d191769
--- /dev/null
+++ b/etc/accounts.yaml.sample
@@ -0,0 +1,7 @@
+- username: 'user_1'
+  tenant_name: 'test_tenant_1'
+  password: 'test_password'
+
+- username: 'user_2'
+  tenant_name: 'test_tenant_2'
+  password: 'test_password'