Fix missing .cfg on exist_debian_interface function check

Glean should not overwrite network interfaces config files
if they already exist, per https://review.openstack.org/#/c/202984.
Glean writes Debian/Ubuntu network interface config files as
/etc/network/interfaces.d/<interface>.cfg, but the code checks
/etc/network/interfaces.d/<interface> prior to writing it.
Missing the .cfg extension on the string makes the check
to always evaluate to false, thus causing Glean to always overwrite
the network config files it may have created previously.
Also, checking the configuration file doesn't exist already
on the block of code for config drive interfaces.

Change-Id: I74bab07d3302752fe94c9182dc63ade25cd98c61

glean/cmd.py

@@ -129,7 +129,7 @@ def write_redhat_interfaces(interfaces, sys_interfaces):
 
 
 def _exists_debian_interface(name):
-    file_to_check = '/etc/network/interfaces.d/{name}'.format(name=name)
+    file_to_check = '/etc/network/interfaces.d/{name}.cfg'.format(name=name)
     return os.path.exists(file_to_check)
 
 
@@ -153,6 +153,9 @@ def write_debian_interfaces(interfaces, sys_interfaces):
             interface_name = "{0}.{1}".format(vlan_raw_device,
                                               interface['vlan_id'])
 
+        if _exists_debian_interface(interface_name):
+            continue
+
         iface_path = os.path.join(eni_d_path, '%s.cfg' % interface_name)
 
         if interface['type'] == 'ipv4_dhcp':

glean/tests/test_glean.py

@@ -106,7 +106,7 @@ class TestGlean(base.BaseTestCase):
                     sample_data_path, sample_prefix,
                     path[1:])
             if path in ['/etc/sysconfig/network-scripts/ifcfg-eth2',
-                        '/etc/network/interfaces.d/eth2']:
+                        '/etc/network/interfaces.d/eth2.cfg']:
                 # Pretend this file exists, we need to test skipping
                 # pre-existing config files
                 return True