update folsom conf template

Running the non-smoke tests revealed that
there were credentials not being set for nova-admin.

This commit sets these credentials to be the existing
keystone credentials.

templates/tempest.folsom.conf.erb

@@ -158,11 +158,11 @@ tenant_name = demo
 # the admin-only parts of the Compute API
 
 # This should be the username of a user WITH administrative privileges
-username = admin
+username = <%= admin_username %>
 # The above administrative user's password
-password = pass
+password = <%= admin_password %>
 # The above administrative user's tenant name
-tenant_name = admin
+tenant_name = <%= admin_tenant_name %>
 
 [network]
 # This section contains configuration options used when executing tests