Add script for enabling offline mode

What does this MR do?

This merge request copies the enable-offline-mode.sh script from the offline-FREEZE branch of the java-maven test project that was added in this merge request. The purpose is to create a unified way of enabling offline mode in test projects.

What are the relevant issue numbers?

• Create shared offline set up script for depende... (gitlab-org/gitlab#433026) • Unassigned • Backlog
• Gemnasium pipeline fails b/c of java-maven@offl... (gitlab-org/gitlab#419571 - closed) • Philip Cunningham • 16.6

Does this MR meet the acceptance criteria?

• [ ] Tests added for this feature/bug