Added maxItems to JSON schema

Closes #593

Here are the results from testing a manifest in staging that contains more than 10 items:

| lint.folder_structure        | INFO     | Validating folder structure
| hardening_manifest           | INFO     | Validating schema
| hardening_manifest           | INFO     | This task will exit if not completed within 2 minutes
| hardening_manifest           | ERROR    | Hardening Manifest failed jsonschema validation
| hardening_manifest           | ERROR    | Verify Hardening Manifest content
| hardening_manifest           | ERROR    | ['8.6', 'latest', 'latest1', 'latest2', 'latest3', 'latest4', 'latest5', 'latest6', 'latest7', 'latest8', 'latest9', 'latest10'] is too long
| lint.dockerfile_validation   | INFO     | Dockerfile is validated.
| lint_jobs                    | WARNING  | Skipping pipeline auth status due to prior failure
| lint_jobs                    | ERROR    | The following stages returned error code: 1
| lint_jobs                    | ERROR    | 	- hardening_manifest_validation
| lint_jobs                    | ERROR    | Failing pipeline

And here is one with an acceptable number:

 lint.folder_structure        | INFO     | Validating folder structure
| hardening_manifest           | INFO     | Validating schema
| hardening_manifest           | INFO     | This task will exit if not completed within 2 minutes
| hardening_manifest           | INFO     | Checking for FIXME values in labels/maintainers
| hardening_manifest           | INFO     | Checking label values
| hardening_manifest           | INFO     | Checking maintainer values
| hardening_manifest           | INFO     | Checking for invalid image sources
| hardening_manifest           | INFO     | Checking image resource sources
| lint.metadata                | INFO     | Hardening manifest is validated
| lint.dockerfile_validation   | INFO     | Dockerfile is validated.
| API.VatAPI                   | INFO     | Checking access to redhat/ubi/ubi8
| API.VatAPI                   | INFO     | [MASKED]/internal/import/check-access/redhat/ubi/ubi8
| lint.pipeline_auth_status    | INFO     | Retrieve Auth Status from VAT
| lint.pipeline_auth_status    | INFO     | Response: {"message":"Access Granted","projectId":9,"imageName":"redhat/ubi/ubi8"}
| lint_jobs                    | INFO     | All stages successful