Support the append argument to the user module

When true, this argument prevents removal of an existing user from groups
not listed in the groups argument.

README.md

@@ -38,6 +38,7 @@ The following attributes are required for each user:
   - 'on_create' will only set the password for newly created users.
 * group - optional primary group override
 * groups - a list of supplementary groups for the user.
+* append - if yes, will only add groups, not set them to just the list in groups (optional).
 * profile - a string block for setting custom shell profiles
 * ssh_key - This should be a list of ssh keys for the user (optional). Each ssh key
   should be included directly and should have no newlines.

tasks/main.yml

@@ -17,6 +17,7 @@
     group: "{{item.group | default(item.username if users_create_per_user_group else users_group)}}"
     # empty string removes user from all secondary groups
     groups: "{{item.groups | join(',') if 'groups' in item else ''}}"
+    append: "{{item.append | default(omit)}}"
     shell: "{{item.shell if item.shell is defined else users_default_shell}}"
     password: "{{item.password if item.password is defined else '!'}}"
     comment: "{{item.name if item.name is defined else ''}}"