generate.py: Enable the detection of deprecated config element

From release to release, some configuration environment my disappear
or get renamed. Schema validation is an option to confirm that the
yaml file is valid, but the generate.py should be able to let the
integrator know which variable is deprecated, hence s/he will be
able to take action upon.

A `deprecated.yaml` will be in present every `openstack-yaml-infra-ref-arch` release
listing the parameters that have been deprecated from previous release to current one.

Something like : 
```yaml
---
oldvalue: newvalue
foo: bar
```

This will trigger the following line to appear if `oldvalue` and `foo` are present in the environment file
```
jdoe@london$ python generate.py 1 global.yml common.yaml.tmpl deprecated.yaml
[DEPRECATED] oldvalue is now deprecated and should be replaced by mynewvalue
[DEPRECATED] foo is now deprecated and should be replaced by bar
```