Added config file validation to clusters/agent.rb

What does this MR do?

New validations have been requested, related to the Clusters::Agent model. This model should now error if an associated config file is missing. See discussion (#232791 (comment 391961294))

The MR will:

• Add a custom validation on the Clusters::Agent model. It checks for an associated config file and errors if the config file is missing.
• Updates related tests

Screenshots

Screen_Recording_2020-08-18_at_12.32.10_PM

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team