nm: iterate the main context when refresh required

In CI, we noticed nmstate might not able to retried applied config
for DHCP interface even with retry. This might caused by outdated cache
in `NM.Client` and cache is not refreshed after simple sleep between
verification.

The fix is iterate the main context till all event processed when
refreshing is issued.

Signed-off-by: Gris Ge <fge@redhat.com>

libnmstate/nm/context.py

@@ -69,6 +69,10 @@ class NmContext:
     def cancellable(self):
         return self._cancellable
 
+    def refresh(self):
+        while self.context.iteration(False):
+            pass
+
     @property
     def client(self):
         if self._quitting:

libnmstate/nm/plugin.py

@@ -191,6 +191,8 @@ class NetworkManagerPlugin(NmstatePlugin):
 
     def refresh_content(self):
         self.__applied_configs = None
+        if self._ctx:
+            self._ctx.refresh()
 
     def apply_changes(self, net_state, save_to_disk):
         NmProfiles(self.context).apply_config(net_state, save_to_disk)