[Share groups] Add scheduler filter ConsistentSnapshotFilter

That will be used for scheduling share groups based on their possibility
to create consistent snapshots.

Also apply following tempest plugin changes:
- Add new 'capability_sg_consistent_snapshot_support' tempest config
option, that will be used for creation of new share group types and used
to prove that scheduling works as expected.
- Fix some share group test attributes from 'only API involved' to
  'API and Backend are involved', because it is so indeed.

Change-Id: I05553c308ae40c4ddc2c6469ff1c1a3da36a87da
Partially-Implements BP manila-share-groups
diff --git a/manila_tempest_tests/config.py b/manila_tempest_tests/config.py
index 7c5eeb2..3be873f 100644
--- a/manila_tempest_tests/config.py
+++ b/manila_tempest_tests/config.py
@@ -112,6 +112,11 @@
                      "capability called 'revert_to_snapshot_support' "
                      "and will be used for setting up custom share type. "
                      "Defaults to the value of run_revert_to_snapshot_tests."),
+    cfg.StrOpt("capability_sg_consistent_snapshot_support",
+               choices=["host", "pool", None],
+               help="Backend capability to create consistent snapshots of "
+                    "share group members. Will be used with creation "
+                    "of new share group types as group spec."),
     cfg.StrOpt("share_network_id",
                default="",
                help="Some backend drivers requires share network "