Make migrate timeout parametrizable

Currently, migrate actions timeout is hard coded to 120 seconds. The helper method in nova_helper is setting 120s by default

https://github.com/openstack/watcher/blob/45cc5b9d8ba9e82d20d21d4b3eabcaf6992b26e2/watcher/common/nova_helper.py#L303

And the migrate action is not exposing the timeout as a parametrizable value

https://github.com/openstack/watcher/blob/45cc5b9d8ba9e82d20d21d4b3eabcaf6992b26e2/watcher/applier/actions/migration.py#L119-L120

I think we should be able to parametrize that timeout. I'd propose to:

1. Add a new parameter

[watcher_applier]
migrate_timeout = X

Default value should be higher than 120s which is too small for usual environments.

The applier services would use that timeout when executing any migrate actions.

2. (optional) Add a new strategy parameter for all the strategies which would define the timeout for migrate actions in action plans created by that audit.

If defined, that would override the system-wide config option for timeout on those specific parameters.