867435 - Importing GPG file with some random text shouldn't be allowed

Adding an validator for GpgKey creation so the imported or pasted string contained a header and footer typical for gpg public keys
e.g. 
-----BEGIN PGP PUBLIC KEY BLOCK-----

GPG KEY

-----END PGP PUBLIC KEY BLOCK-----

GPG key itself should not be longer than 65 characters per line and should contain only base64 characters.

Also added attribute to the katello_default.yml 
https://github.com/Katello/katello/pull/2956/files#L2R29

so now the gpg key validation strictness is optional