Force overwrite on cred_store tests

Certain versions of krb5 (>= 1.14 or anything that backported the
patch, including RHEL/Centos/Fedora) object (probably rightly so) to
duplicate credentials already existing in the ccache.  Tell those
versions to sit down and shut up.

This is the third and final commit in a series to enable running our
test suite on RHEL-7.

gssapi/tests/test_high_level.py

@@ -173,7 +173,8 @@ def test_store_acquire(self):
         deleg_creds = server_ctx.delegated_creds
         deleg_creds.shouldnt_be_none()
 
-        store_res = deleg_creds.store(usage='initiate', set_default=True)
+        store_res = deleg_creds.store(usage='initiate', set_default=True,
+                                      overwrite=True)
         store_res.usage.should_be('initiate')
         store_res.mechs.should_include(gb.MechType.kerberos)
 

gssapi/tests/test_raw.py

@@ -475,7 +475,7 @@ def test_store_cred_acquire_cred(self):
         deleg_creds = server_ctx_resp.delegated_creds
         deleg_creds.shouldnt_be_none()
         store_res = gb.store_cred(deleg_creds, usage='initiate',
-                                  set_default=True)
+                                  set_default=True, overwrite=True)
 
         store_res.shouldnt_be_none()
         store_res.usage.should_be('initiate')