Use empty env variable set if missing

When a set of global_environment_variables is not provided, Ansible
throws a warning:

  [WARNING]: could not parse environment value, skipping:
  [u'{{deployment_environment_variables | default({}) }}',
  u'{{global_environment_variables | default(omit) }}']

This patch returns an empty dict rather than using the omit filter.

Change-Id: I5e3399b816779821e936b84a2972ca1db39c1a62

tasks/repo_clone_git.yml

@@ -34,7 +34,7 @@
 - name: Clone git repositories asynchronously
   become: yes
   become_user: "{{ repo_build_service_user_name }}"
-  environment: "{{ global_environment_variables | default(omit) }}"
+  environment: "{{ global_environment_variables | default({}) }}"
   git:
     repo: "{{ item['url'] }}"
     dest: "{{ repo_build_git_dir }}/{{ item['name'] }}"