Clean up notification settings in glance-api.conf

Currently, rabbitmq settings in glance-api.conf are hard-coded and
overrides to rabbitmq settings in environment will not be reflected in
configuration file.  This change addresses that while also only writing
rabbitmq/qpid settings when an appropriate notification strategy is
selected.

Change-Id: I9d06d4b1e030f14293d143021bd78d732adafcc2
Closes-Bug: #1273020
Merge "Clean up notification settings in glance-api.conf"