Clear pexpect buffer before attempting to disconnect.

Sometimes the buffer was not cleared and this would lead to longer disconnect times
diff --git a/TestON/drivers/common/cli/onosdriver.py b/TestON/drivers/common/cli/onosdriver.py
index 3d9debf..2e7ffa3 100644
--- a/TestON/drivers/common/cli/onosdriver.py
+++ b/TestON/drivers/common/cli/onosdriver.py
@@ -82,6 +82,8 @@
         '''
         response = ''
         try:
+            self.handle.sendline("\n")
+            self.handle.expect("\$")
             self.handle.sendline("exit")
             self.handle.expect("closed")
         except pexpect.EOF:
@@ -1014,10 +1016,3 @@
 
 
 
-
-
-
-
-
-
-