[TrivialFix] Correct file mode settings

Drop the undesired executable bit for the following files:
  1. manila_tempest_tests/services/share/v2/json/shares_client.py
  2. manila_tempest_tests/tests/api/test_share_networks.py

Also add pep8 check for new executable files under manila_tempest_tests.

Change-Id: Iaa509fdf4e0ebeb60d71b053f62459992c938468
diff --git a/manila_tempest_tests/services/share/v2/json/shares_client.py b/manila_tempest_tests/services/share/v2/json/shares_client.py
old mode 100755
new mode 100644
diff --git a/manila_tempest_tests/tests/api/test_share_networks.py b/manila_tempest_tests/tests/api/test_share_networks.py
old mode 100755
new mode 100644