Allow to run DIB when installed system-wide
Allow to run DIB when installed system-wide

This will allow to package the script and install it system-wide

In that case, the following configuration is expected:
* diskimage-builder installed in default system-wide location
* TripleO elements installed in _prefix_/tripleo-image-elements
* Octavia elements installed in _prefix_/octavia-image-elements
With possible paths for _prefix_: /usr/share or /usr/local/share

These will be used as a fallback, after local checkout directories

Change-Id: I021530b731f7a98043ea205b7014e71b39dbc8e3

openstack/octavia,openstack/octavia,openstack/octavia