Add scm_branch while preloading data

Currently, the SCM url for default project is not able to
fetch playbooks as branch is not specified. So, adding the
scm_branch value resolves this issue.

Signed-off-by: Vismay Golwala <vgolwala@redhat.com>

awx/main/management/commands/create_preload_data.py

@@ -30,6 +30,7 @@ class Command(BaseCommand):
                 p = Project(name='Demo Project',
                             scm_type='git',
                             scm_url='https://github.com/ansible/ansible-tower-samples',
+                            scm_branch='master',
                             scm_update_on_launch=True,
                             scm_update_cache_timeout=0,
                             organization=o)