Fix inconsistent type of default value

The provider_net_base_segmentation_id parameter is StrOpt, thus expects
a string value. This change fixes the incorrect type(integer) of
its default value to get rid of the following warning message.

/usr/lib/python3.6/site-packages/oslo_config/types.py:59: UserWarning:
 converting '3000' to a string
  warnings.warn('converting \'%s\' to a string' % str_val)

Closes-Bug: #1959859
Change-Id: I92aeb04f6c5aefd26e83b83c17d80341418aa233
diff --git a/tempest/config.py b/tempest/config.py
index a840a97..7c81dd2 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -873,7 +873,7 @@
     cfg.StrOpt('qos_placement_physnet', default=None,
                help='Name of the physnet for placement based minimum '
                     'bandwidth allocation.'),
-    cfg.StrOpt('provider_net_base_segmentation_id', default=3000,
+    cfg.StrOpt('provider_net_base_segmentation_id', default='3000',
                help='Base segmentation ID to create provider networks. '
                     'This value will be increased in case of conflict.'),
     cfg.BoolOpt('qos_min_bw_and_pps', default=False,