Validate Kubernetes CA certificate format

OpenSSL::X509::CertificateError: nested asn1 error

https://sentry.gitlab.net/gitlab/gitlabcom/issues/558463/

OpenSSL::X509::CertificateError: nested asn1 error
  clusters/platforms/kubernetes.rb:185:in `initialize'
    opts[:cert_store].add_cert(OpenSSL::X509::Certificate.new(ca_pem))
  clusters/platforms/kubernetes.rb:185:in `new'
    opts[:cert_store].add_cert(OpenSSL::X509::Certificate.new(ca_pem))
  clusters/platforms/kubernetes.rb:185:in `kubeclient_ssl_options'
    opts[:cert_store].add_cert(OpenSSL::X509::Certificate.new(ca_pem))
  clusters/platforms/kubernetes.rb:164:in `build_kube_client!'
    ssl_options: kubeclient_ssl_options,
  clusters/platforms/kubernetes.rb:171:in `read_pods'
    kubeclient = build_kube_client!
...
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nested asn1 error