Update userDisplayNameField to use "displayName" instead of "gecos"

[stephdl]

This pull request updates the userDisplayNameField in the configure-module/20config file to use the "displayName" field instead of the "gecos" field.

However

• when the user has already login, it does not honor the new setting because it has been written to postgres

webtop5=# SELECT * FROM core.users;
 domain_id  |   user_id    | type | enabled |               user_uid               |      display_name       |      se
cret      
------------+--------------+------+---------+--------------------------------------+-------------------------+--------
----------
 NethServer | john     | U    | t       | 24e4751b-fc50-4d01-8c65-107a98b7434a | de labrusse             | 3c6sm72
wdpuw22sg
 NethServer | foo       | U    | t       | 3beecb12-63e3-422f-b570-405277fd8cbf | de labrusse             | cqw7q6z
2ydn3i2es
 *          | admin        | U    | t       | 991f72dc-2b96-4340-b88f-53506b160519 | Administrator           | 76gvj7v
bpnuhsf6r
(7 rows)

• when you save the configuration after the update of the module, it is not taken except if you change the user domain or the mail server

so in short it is valid only for first installation

NethServer/dev#6923

[DavidePrincipi]

LGTM

[DavidePrincipi]

@Amygos is this fix effective also for already provisioned instances?

[Amygos]

@Amygos is this fix effective also for already provisioned instances?

No, the fix will apply only on fresh module installations, or when the value of mail_module and/ormail_domain changes, or after a module move/clone.
Also as pointed out by @stephdl the new value will be applied only for the users that log in for the first time.

To avoid the first issue we can resume the work in the PR #19, so we can always set the correct value in the configure-module action or on the module update.

For the users already created on db, I would leave the old value of the filed.

[stephdl]

noobs question @Amygos, what is the purpose or the advantage of your solution ?