feat(dashmate): validate SSL certificate files

[shumkov]

Issue being fixed or feature implemented

It appeared as a common problem that people do not include server certificates in certificate chain file

What was done?

• Added explanation to include server certificate in certificate chain file
• Validate that certificate chain file corresponds to private key

How Has This Been Tested?

Passing valid and invalid cert files to dashmate setup.

Breaking Changes

None

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone