Explicitly set default notifier strategy

Havana claims to set this to the default, but in reality it does not.
The default is actually set to qpid because oslo.config automatically
looks for default options in /usr/share/glance/glance-api-dist.conf,
which in turn sets `notifier_strategy = qpid`.

Later version of oslo.config seem to have fixed this behavior and do not
load *-dist* config files.

Though this patch exposes a $notifier_strategy parameter to the
glance::api class, including glance::notify::{rabbit,qpid} to set the
correct notifier strategy should still be the prescribed way to override
the 'noop' behavior.

Change-Id: Ic2c67da463e7053a98042d9cba099aa857913ee7
Merge "Explicitly set default notifier strategy" into stable/havana