Do not assume ca_key_path is a RSA key

[Lasering]

Cookbook Version

2.1.0

Scenario

Currently the cookbook assumes ca_key_path is a RSA key, which isn't always true.
https://github.com/zuazo/ssl_certificate-cookbook/blob/master/libraries/resource_ssl_certificate_generators.rb#L168

def generate_ca_from_content(cert_content, key_content)
  ca_cert = OpenSSL::X509::Certificate.new(cert_content)
  ca_key = OpenSSL::PKey::RSA.new(key_content, ca_key_passphrase)
  [ca_cert, ca_key]
end

This could be easily fixed with:

  ca_key = OpenSSL::PKey.read(key_content, ca_key_passphrase)

https://ruby.github.io/openssl/OpenSSL/PKey.html