Add support for global dimensions
Add support for global dimensions

If global dimensions are specified at top level, then do not
require per log entry level dimensions. If both global and
per log level dimensions are provided, then the per log level
dimensions will override the global one.

Change-Id: I4576383e4182916953d18dce55a65737116e1f74
implements: https://review.openstack.org/#/c/273058

openstack/monasca-log-api,openstack/monasca-log-api,stackforge/monasca-log-api,stackforge/monasca-log-api,openstack/monasca-log-api,stackforge/monasca-log-api