Tempest: add a Keystone reseller feature flag

A new tempest test is being added in https://review.openstack.org/#/c/285541/
but it is not supported in the Kilo and Liberty branches. This patch
turns on this feature flag at Devstacks's side.

According to tempest policies, this patch must be merged first so the
test can actually run.

Change-Id: I52458a0b36e1dba233667311b35f6c3931e2e66c
Depends-On: Ie69dae09c2b42e825e9d51abf158fc14788387d1
diff --git a/lib/tempest b/lib/tempest
index d4d8cf2..ee45501 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -282,6 +282,10 @@
         iniset $TEMPEST_CONFIG identity ca_certificates_file $SSL_BUNDLE_FILE
     fi
 
+    # Identity Features
+    # TODO(rodrigods): Remove the reseller flag when Kilo and Liberty are end of life.
+    iniset $TEMPEST_CONFIG identity-feature-enabled reseller True
+
     # Image
     # We want to be able to override this variable in the gate to avoid
     # doing an external HTTP fetch for this test.