PDF: Run YAML Linter on pod descriptors / output

PDF: Run YAML Linter on pod descriptors / output

We have 2 rounds of YAML files:
- PDF (input file(s) for the installer adapter templates);
- parsed PDF (output file(s) after installer adapter template parse);

Run yamllint on all these files, and summarize the output in a tuple
form: (valid YAML output, successful parse, installer templates cnt).

This helps catching various issues:
- formatting issues in installer adapter templates;
- missing values in PDFs (e.g. lf-pod4 IPMI credentials);
etc.

For now, yamllint failures for output files are non-fatal.

Sample output:

| PDF Verify Matrix        | YAML Lint  | ... | fuel   | joid   |
| pharos/config/pod1.yaml  | OK         | ... | 1/1/1  | 1/1/1  |
| intel/pod18.yaml         | OK         | ... | 1/1/1  | 1/1/1  |
| arm/pod5.yaml            | OK         | ... | 1/1/1  | 0/1/1  |
| lf/pod4.yaml             | OK         | ... | 0/1/1  | 0/1/1  |

Change-Id: Id598da89fab0e7e41641649833471194e8d248a9
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>