community cluster pre-reqs

Create groovy scripts to install any Jenkins pre-reqs such as plugins and credentials.

To create/add a new credential:

• Add credentials into the repo, such as ios certs: https://github.com/fheng/help/blob/bfdffe83d09a469486e0795bc9183337b7a6b3b5/developer_guides/clientsdk/7.buildfarm_ios_certificates.md
• Create a groovy script to create a specific credential, exemple: https://github.com/fheng/jenkins-bob-builder/blob/master/scripts/add_ssh_credentials.groovy
• Invoke the new script in https://github.com/fheng/jenkins-bob-builder/blob/master/scripts/configure_jenkins.sh

Some plugins that may need to be installed:

• android-signing
• xcode-plugin