Enhance Savanna discovery logic

Two new parameters could be added to the Horizon's local_settings.py:
SAVANNA_SERVICE - the UI searches in keystone for service having that name. Set to 'mapreduce' by default.
SAVANNA_URL - if present, UI accesses Savanna by that URL, ignoring SAVANNA_SERVICE.

Change-Id: I8a7808a05a707259f67f43370bcfba40a0609663