Delete REST client active sessions in CI

During long boot test, the REST call to check on boot progress failed
where server failed to respond due to too many session opened. Refer
openbmc/openbmc#1389

Ran test with this fix and it pass the test now.

Resolves openbmc/openbmc-test-automation#464

Change-Id: I0a0e0929749f2698295ae5ece64de3ea05043210
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/tests/test_basic_poweron.robot b/tests/test_basic_poweron.robot
index 2a8a445..f8a75aa 100644
--- a/tests/test_basic_poweron.robot
+++ b/tests/test_basic_poweron.robot
@@ -45,3 +45,5 @@
 
     Initiate Host Boot
     Wait Until Keyword Succeeds  10 min  10 sec  Is OS Starting
+
+    Flush REST Sessions