The question is simple: what is the difference between ansible_user (former ansible_ssh_user) and remote_user in Ansible, besides that the first one is set if configuration file and the latter one is set in plays / roles? How do they relate to -u / --user command line options?
They both seem to be the same.
Take a look here:
# the magic variable mapping dictionary below is used to translate
# host/inventory variables to fields in the PlayContext
# object. The dictionary values are tuples, to account for aliases
# in variable names.
MAGIC_VARIABLE_MAPPING = dict(
connection = ('ansible_connection',),
remote_addr = ('ansible_ssh_host', 'ansible_host'),
remote_user = ('ansible_ssh_user', 'ansible_user'),
port = ('ansible_ssh_port', 'ansible_port'),
Source: https://github.com/ansible/ansible/blob/c600ab81ee/lib/ansible/playbook/play_context.py#L46-L55
Besides, ansible_user is used when we want to specifiy default SSH user in ansible hosts file where as remote_user is used in playbook context.
From https://github.com/ansible/ansible/blob/c600ab81ee/docsite/rst/intro_inventory.rst
ansible_user The default ssh user name to use.
and here is an example of using ansible_user in ansible hosts file:
[targets]
localhost ansible_connection=local
other1.example.com ansible_connection=ssh ansible_user=mpdehaan
other2.example.com ansible_connection=ssh ansible_user=mdehaan