This fixes the --ignore-missing-schemas flag

[ian-howell]

Addresses #149

This changes the behavior of the --ignore-missing-schemas flag. Prior to
this change, the flag prevented validation against schemas for all
documents. The new behavior is to prevent validation against schemas for
any document whose associated schema can not be found. All other
documents are validated against their corresponding schemas.

This also improves the expected behavior of the flag. Prior behavior was
to display a successful message for any document missing its schema. New
behavior will print a warning message that the document was not
validated against a schema. Note that this still returns a success code.

The following demostrates the new behavior against the valid.yaml, invalid.yaml, and test_crd.yaml files located in the fixtures directory:

$ kubeval fixtures/valid.yaml
The document fixtures/valid.yaml contains a valid ReplicationController

$ kubeval fixtures/valid.yaml --ignore-missing-schemas
Warning: Set to ignore missing schemas
The document fixtures/valid.yaml contains a valid ReplicationController


$ kubeval fixtures/invalid.yaml
The document fixtures/invalid.yaml contains an invalid ReplicationController
---> spec.replicas: Invalid type. Expected: integer, given: string

$ kubeval fixtures/invalid.yaml --ignore-missing-schemas
Warning: Set to ignore missing schemas
The document fixtures/invalid.yaml contains an invalid ReplicationController
---> spec.replicas: Invalid type. Expected: integer, given: string


$ kubeval fixtures/test_crd.yaml
1 error occurred:
        * Failed initalizing schema https://kubernetesjsonschema.dev/master-standalone/sealedsecret-bitnami-v1alpha1.json: Could not read schema from HTTP, response status is 404 Not Found

$ kubeval fixtures/test_crd.yaml --ignore-missing-schemas
Warning: Set to ignore missing schemas
The document fixtures/test_crd.yaml containing a SealedSecret was not validated against a schema

[mikespokefire]

Can confirm this fixes the bug, thanks for sorting that out @ian-howell! 🎉