Change tempest BadRequest exc to tempest-lib exc

This commit changes tempest.exceptions.BadRequest to
tempest_lib.exceptions.BadRequest. This is one of the migrating rest
client to tempest-lib works.

Change-Id: I859bd25c07c6344e6455c4010c253cbdf04315b3
diff --git a/tempest/api/network/base.py b/tempest/api/network/base.py
index 4cb1485..e8c8de3 100644
--- a/tempest/api/network/base.py
+++ b/tempest/api/network/base.py
@@ -223,7 +223,7 @@
                     gateway_ip=gateway_ip,
                     **kwargs)
                 break
-            except exceptions.BadRequest as e:
+            except lib_exc.BadRequest as e:
                 is_overlapping_cidr = 'overlaps with another subnet' in str(e)
                 if not is_overlapping_cidr:
                     raise