Add config validation option

Add a configuration validation command-line option.  This will parse
the yaml file -- sufficient to find many indenting errors -- and then
run the file through a schema check.  There is one example of checking
image/provider mappings; more such validation can be added.

Test-cases are updated

The intent is to use this within a gate-job for system-config to avoid
broken configs being committed.

Change-Id: I250ea4959740cfc4650e9057caba09ae7bc25768

requirements.txt

@@ -17,3 +17,4 @@ MySQL-python
 PrettyTable>=0.6,<0.8
 six>=1.7.0
 diskimage-builder
+voluptuous