Handle 'groups=' (empty string) in user module (Linux)

Makes it possible to remove a user from all but the primary group.

Setting 'groups=' is the equivalent to passing '-G ""' to
useradd/usermod, which is interpreted as "no group" on Linux.

user

@@ -48,7 +48,9 @@ options:
     groups:
         required: false
         description:
-            - Puts the user in this comma-delimited list of groups.
+            - Puts the user in this comma-delimited list of groups. When set to
+              the empty string ('groups='), the user is removed from all groups
+              except the primary group.
     append:
         required: false
         description:
@@ -249,6 +251,7 @@ class User(object):
             cmd.append(self.group)
 
         if self.groups is not None:
+            if self.groups != '':
                 for g in self.groups.split(','):
                     if not self.group_exists(g):
                         self.module.fail_json(msg="Group %s does not exist" % (g))
@@ -301,13 +304,19 @@ class User(object):
 
         if self.groups is not None:
             current_groups = self.user_group_membership()
+            groups_need_mod = False
+            groups = []
+
+            if self.groups == '':
+                if current_groups and not self.append:
+                    groups_need_mod = True
+            else:
                 groups = self.groups.split(',')
                 for g in groups:
                     if not self.group_exists(g):
                         self.module.fail_json(msg="Group %s does not exist" % (g))
 
                 group_diff = set(sorted(current_groups)).symmetric_difference(set(sorted(groups)))
-            groups_need_mod = False
 
                 if group_diff:
                     if self.append: