Use the new oslo.report config option for specifying a
log_dir in which the GMR reports can be generated in.
consolidate the gmr related setup code in config.py
Introduced in oslo.reports change id:
I9ed0191628f2d552acd8130687b636671bc07a3e
Change-Id: Ib5edf65fd33ce86757f7cb1a6546a6f00738c9a3