Add a common generic dynamic loader for watcher

In watcher, an audit generates a set of actions which
aims at achieving a given goal (lower energy consumption, ...).
It is possible to configure different strategies in order to achieve
each goal. Each strategy is written as a Python class which produces
a set of actions. Today, the set of possible actions is fixed for a
given version of Watcher and enables optimization algorithms to
include actions such as instance migration, changing hypervisor state,
changing power state (ACPI level, ...).

This patchset add a common generic dynamic loader for plugins,
such as for custom Actions, Strategies, Planners, etc.

Partially implements: blueprint watcher-add-actions-via-conf

Change-Id: I59d031b93865fff2540e3973921e1bdafa95f88e

watcher/common/exception.py

@@ -297,3 +297,7 @@ class VMNotFound(WatcherException):
 
 class HypervisorNotFound(WatcherException):
     message = _("The hypervisor could not be found")
+
+
+class LoadingError(WatcherException):
+    message = _("Error loading plugin '%(name)s'")