Deal with different django-admin executables

After the recent patch [1] the devstack is broken for Fedora 20 with
the error "django-admin.py: command not found" during horizon setup.

This is due to differences in how django currently packaged for Fedora,
where we should use "django-admin", without the .py

This patch sets up executable alias by checking if "django-admin"
exists, and falling back on "django-admin.py".

[1] https://review.openstack.org/#/c/120940/

Change-Id: I2b6de25fe32446edbdc0418674fea8579ec739d9
Merge "Deal with different django-admin executables"