This commit removes the regular expression check on OIDC usernames.
Although it is not recommended to use any character in a username,
it is possible to create and use them. The tool useradd has the flag
--badname and adduser has --allow-badname and --allow-all-names to
create new users with any character.
Moreover, it is possible to create any username with the rest of
provisioners.
Fixes#1436
