[compute]Use single long task executor

Move the execution of build_and_run_instance and snapshot_instance to
one common long task executor. Originally snapshot ran
on the RPC pool, build_and_run_instance ran on the default pool.
Also each of these tasks had a separate concurrency limit enforced by a
semaphore.

After this patch each of these tasks use a common Executor. The size of
that executor and the way how we limit the concurrency differs in
eventlet and in native threading mode.

In eventlet mode we have one big Executor with "unlimit" size and
individual semaphores are used for each task type to enforce the
configured limits.

In threading mode we requests the admin to configure the 2 limits to the
same number, and we warn if not. We use that limit (or the max of the 2
limits) as the size of the long task Executor. As the limits are the
same we don't enforce individual limit any more. The executor size will
ensure the shared limit is kept. As the limit is shared a single
operation type can consume the whole limit.

Note that while live migration is a long-running task we cannot put it into
the same long_task_executor as build and snapshot as we need:
1. a very small limit of concurrent live migrations compared to
   builds and snapshots
2. a way to cancel live migrations easily that are waiting due to the
   limit

Change-Id: I88a6a593af8a5b518715e1245a76ee54752afe83
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

releasenotes/notes/deprecate-unlimited-max_concurrent_live_migrations-29c54c7eeb77041c.yaml

@@ -36,6 +36,14 @@ upgrade:
     now reduced to 5 native threads. Please also read the `concurrency
     <https://docs.openstack.org/nova/latest/admin/concurrency.html>`__
     guide for more details.
+  - |
+    In native threading mode the limit expressed by
+    ``[DEFAULT]max_concurrent_builds`` and
+    ``[DEFAULT]max_concurrent_snapshots`` configuration options are shared
+    across the two operation types as they are executed by the same Executor.
+    Therefore the two config options need to be set to the same value. If not
+    then nova will use the bigger value as the shared limit. The shared limit
+    also means that one operation type can consume the whole limit.
 deprecations:
   - |
     The possible 0 value of the configuration option