Add horizon plugin configuration file

Implements blueprint murano-as-horizon-plugin
* Use prepare_murano.sh to add murano to the horizon automatically
* Or add _50_murano.py manually under openstack_dashboard/enabled directory
* settings.py is no longer tracked and needed to be syncronized
with current horizon version
* Murano panel order can be changed by renaming prefix _50 to the desired positon

Change-Id: I26e15bae5d62fba2f83d8792e888897674cab8e1
Merge "Add horizon plugin configuration file"