build: fix python load paths

Fixed python load paths so that vppcfg will work installed as python
library and standalone from the source directory, fixing load
pathes for resources such as yaml files along the way.

Added a make target for pylint called 'make check-style', fixed a
number of minor pylint issues along the way.

Signed-off-by: Ray Kinsella <mdr@ashroe.eu>

docs/config-guide.md

@@ -13,7 +13,7 @@ types of validation:
     to a running VPP. *Note*: Some semantic checks are stricter than VPP, because applying
     them may leave the dataplane in a non-recoverable state.
 
-For the curious, the Yamale syntax validation lives in [this schema](../schema.yaml).
+For the curious, the Yamale syntax validation lives in [this schema](../vppcfg/schema.yaml).
 If you want to get started quickly and don't mind cargo-culting, take a look at [this example](../example.yaml).
 
 ### Basic structure