Finally got it figured out. It's "driver options", not "pdo attributes".

  $databases = array (
    'default' => 
     array (
       'default' => 
        array (
          'database' => 'drupaldb',
          'username' => 'drupaluser',
          'password' => 'password',
          'host' => '192.168.10.100',
          'port' => '3306',
          'driver' => 'mysql',
          'prefix' => '',
          'driver options' => array(
              PDO::MYSQL_ATTR_SSL_KEY => '/path/client-key.pem',
              PDO::MYSQL_ATTR_SSL_CERT => '/path/client-cert.pem',
              PDO::MYSQL_ATTR_SSL_CA => '/path/ca-cert.pem',
           ),
       ),
    ),
  );

Edit: Fixed typo.

Finally got it figured out. It's "driver options", not "PDO attributes".

  $databases = array (
    'default' => 
     array (
       'default' => 
        array (
          'database' => 'drupaldb',
          'username' => 'drupaluser',
          'password' => 'password',
          'host' => '192.168.10.100',
          'port' => '3306',
          'driver' => 'mysql',
          'prefix' => '',
          'driver options' => array(
              PDO::MYSQL_ATTR_SSL_KEY => '/path/client-key.pem',
              PDO::MYSQL_ATTR_SSL_CERT => '/path/client-cert.pem',
              PDO::MYSQL_ATTR_SSL_CA => '/path/ca-cert.pem',
           ),
       ),
    ),
  );

Finally got it figured out. It's "driver options", not "pro attributes"

  $databases = array (
    'default' => 
     array (
       'default' => 
        array (
          'database' => 'drupaldb',
          'username' => 'drupaluser',
          'password' => 'password',
          'host' => '192.168.10.100',
          'port' => '3306',
          'driver' => 'mysql',
          'prefix' => '',
          'driver options' => array(
              PDO::MYSQL_ATTR_SSL_KEY => '/path/client-key.pem',
              PDO::MYSQL_ATTR_SSL_CERT => '/path/client-cert.pem',
              PDO::MYSQL_ATTR_SSL_CA => '/path/ca-cert.pem',
           ),
       ),
    ),
  );

Finally got it figured out. It's "driver options", not "pdo attributes".

  $databases = array (
    'default' => 
     array (
       'default' => 
        array (
          'database' => 'drupaldb',
          'username' => 'drupaluser',
          'password' => 'password',
          'host' => '192.168.10.100',
          'port' => '3306',
          'driver' => 'mysql',
          'prefix' => '',
          'driver options' => array(
              PDO::MYSQL_ATTR_SSL_KEY => '/path/client-key.pem',
              PDO::MYSQL_ATTR_SSL_CERT => '/path/client-cert.pem',
              PDO::MYSQL_ATTR_SSL_CA => '/path/ca-cert.pem',
           ),
       ),
    ),
  );

Edit: Fixed typo.