target: poll needed resources only when activating

No need to wait for resources that aren't used when activating a
specific resource.

This reduces invocation times of various operations significantly (at
least in our case):

- labgrid-client power get
    3.2 -> 1.2 s

- labrid-client io get ign
    2.7 -> 1.1 s

- labgrid-client ssh true
    4.2 -> 3.1 s

Signed-off-by: Martin Hundebøll <martin@geanix.com>

Author: hundeboll

labgrid/target.py

@@ -41,15 +41,15 @@ def interact(self, msg):
         else:
             input(msg)
 
-    def update_resources(self):
+    def update_resources(self, resources=None):
         """
         Iterate over all relevant resources and deactivate any active but
         unavailable resources.
         """
         if (monotonic() - self.last_update) < 0.1:
             return
         self.last_update = monotonic()
-        for resource in self.resources:
+        for resource in self.resources if resources is None else resources:
             resource.poll()
             if not resource.avail and resource.state is BindingState.active:
                 deactivated = self.deactivate(resource)
@@ -71,7 +71,7 @@ def await_resources(self, resources, timeout=None, avail=True):
             timeout (float): optional timeout
             avail (bool): optionally wait until the resources are unavailable with avail=False
         """
-        self.update_resources()
+        self.update_resources(resources)
 
         waiting = set(r for r in resources if r.avail != avail)
         static = set(r for r in waiting if r.get_managed_parent() is None)
@@ -102,7 +102,7 @@ def await_resources(self, resources, timeout=None, avail=True):
                 filter=waiting
             )
 
-        self.update_resources()
+        self.update_resources(resources)
 
     def get_resource(self, cls, *, name=None, wait_avail=True):
         """