Always return a sorted list of drivers for configs

glance.store entry point used in Glance for the store library is
resulting in generating random order in each run of the ``tox -e
genconfig`` command. This is because of the unsorted iterator returned
by the stevedore extension manager [1, 2].

This commit fixes that and returs a deterministic, sorted list of
drivers to generate the configs in Glance.

Example scenario is at change Iea2c538b37182191445cc5c1f8b2d9dceed06343

Closes-Bug: 1619487

[1] 22fa2b08e7/glance_store/backend.py (L135)
[2] http://docs.openstack.org/developer/stevedore/managers.html#stevedore.extension.ExtensionManager.__iter__

Co-Authored-By: Joshua Harlow <jxharlow@godaddy.com>

Change-Id: I3146b8597a5d89da49b84d6653edacc3067c2c71

glance_store/backend.py

@@ -132,7 +132,7 @@ def _list_opts():
     driver_opts = []
     mgr = extension.ExtensionManager('glance_store.drivers')
     # NOTE(zhiyan): Handle available drivers entry_points provided
-    drivers = [ext.name for ext in mgr]
+    drivers = sorted([ext.name for ext in mgr])
     handled_drivers = []  # Used to handle backwards-compatible entries
     for store_entry in drivers:
         driver_cls = _load_store(None, store_entry, False)

releasenotes/notes/sorted-drivers-for-configs-a905f07d3bf9c973.yaml

@@ -0,0 +1,16 @@
+---
+prelude: >
+    Return list of store drivers in sorted order for
+    generating configs. More info in ``Upgrade Notes``
+    and ``Bug Fixes`` section.
+upgrade:
+  - This version of glance_store will result in Glance
+    generating the configs in a sorted (deterministic)
+    order. So, preferably store releases on or after this
+    should be used for generating any new configs if the
+    mismatched ordering of the configs results in an issue
+    in your environment.
+fixes:
+  - Bug 1619487 is fixed which was causing random order of
+    the generation of configs in Glance. See ``upgrade``
+    section for more details.