use rolling deploys for beanstalk, instead of the 'all at once' defau…

…lt. The all at once default takes your app offline as documented here: 'https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/using-features.rolling-version-deploy.html?shortFooter=true'. Followed the policy syntax here: 'https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/command-options-general.html?shortFooter=true#command-options-general-elasticbeanstalkcommand'.

handel/src/services/beanstalk/index.ts

@@ -119,6 +119,11 @@ async function getCompiledBeanstalkTemplate(stackName: string, preDeployContext:
     // Configure rolling updates
     handlebarsParams.optionSettings.push(getEbConfigurationOption('aws:autoscaling:updatepolicy:rollingupdate', 'RollingUpdateEnabled', true));
 
+    // Configure rolling deploys
+    handlebarsParams.optionSettings.push(getEbConfigurationOption('aws:elasticbeanstalk:command', 'DeploymentPolicy', 'RollingWithAdditionalBatch'));
+    handlebarsParams.optionSettings.push(getEbConfigurationOption('aws:elasticbeanstalk:command', 'BatchSizeType', 'Percentage'));
+    handlebarsParams.optionSettings.push(getEbConfigurationOption('aws:elasticbeanstalk:command', 'BatchSize', '25'));
+
     // Configure VPC
     handlebarsParams.optionSettings.push(getEbConfigurationOption('aws:ec2:vpc', 'VPCId', accountConfig.vpc));
     handlebarsParams.optionSettings.push(getEbConfigurationOption('aws:ec2:vpc', 'Subnets', accountConfig.private_subnets.join(',')));