[displaymanager] Refactor greeter setup

- Only lightdm has support for dynamically configuring the
   greeter based on what is installed. Still, refactory that
   to something general.

src/modules/displaymanager/main.py

@@ -129,6 +129,14 @@ class DisplayManager(metaclass=abc.ABCMeta):
         """
         # Many implementations do nothing
 
+    @abc.abstractmethod
+    def greeter_setup(self):
+        """
+        Additional setup for the greeter.
+        """
+        # Most implementations do nothing
+
+
 class DMmdm(DisplayManager):
     name = "mdm"
     executable = "mdm"
@@ -514,6 +522,38 @@ class DMlightdm(DisplayManager):
                 )
             )
 
+    def greeter_setup(self):
+        lightdm_conf_path = os.path.join(
+            root_mount_point, "etc/lightdm/lightdm.conf"
+            )
+
+        # configure lightdm-greeter
+        greeter_path = os.path.join(
+            root_mount_point, "usr/share/xgreeters"
+            )
+
+        if (os.path.exists(greeter_path)):
+            # configure first found lightdm-greeter
+            for entry in os.listdir(greeter_path):
+                if entry.endswith('.desktop'):
+                    greeter = entry.split('.')[0]
+                    libcalamares.utils.debug(
+                        "found greeter {!s}".format(greeter)
+                        )
+                    os.system(
+                        "sed -i -e \"s/^.*greeter-session=.*"
+                        "/greeter-session={!s}/\" {!s}".format(
+                            greeter,
+                            lightdm_conf_path
+                            )
+                        )
+                    libcalamares.utils.debug(
+                        "{!s} configured as greeter.".format(greeter)
+                        )
+                    break
+            else:
+                return ("No lightdm greeter installed.")
+
 
 class DMslim(DisplayManager):
     name = "slim"
@@ -711,36 +751,7 @@ def run():
     # setup lightdm
     if "lightdm" in displaymanagers:
         if have_dm("lightdm", root_mount_point):
-            lightdm_conf_path = os.path.join(
-                root_mount_point, "etc/lightdm/lightdm.conf"
-                )
-
-            # configure lightdm-greeter
-            greeter_path = os.path.join(
-                root_mount_point, "usr/share/xgreeters"
-                )
-
-            if (os.path.exists(greeter_path)):
-                # configure first found lightdm-greeter
-                for entry in os.listdir(greeter_path):
-                    if entry.endswith('.desktop'):
-                        greeter = entry.split('.')[0]
-                        libcalamares.utils.debug(
-                            "found greeter {!s}".format(greeter)
-                            )
-                        os.system(
-                            "sed -i -e \"s/^.*greeter-session=.*"
-                            "/greeter-session={!s}/\" {!s}".format(
-                                greeter,
-                                lightdm_conf_path
-                                )
-                            )
-                        libcalamares.utils.debug(
-                            "{!s} configured as greeter.".format(greeter)
-                            )
-                        break
-                else:
-                    return ("No lightdm greeter installed.")
+            pass
         else:
             libcalamares.utils.debug("lightdm selected but not installed")
             displaymanagers.remove("lightdm")