Update install.yml

[gpapadoka]

Trashbin retention 35 days

[aptiko]

Since team-collaboration-toolkit is not specifically for us, but for general use, it doesn't seem right to hardwire our own special requirements. Since this parameter already has a default in nextcloud, "auto", we shouldn't specify another default here. What we should do instead is add a nextcloud_trashbin_retention_obligation parameter to the nextcloud role, which by default will have the same value as the upstream default, i.e. "auto", and use that parameter in our own Ansible playbook.

(Incidentally, I think I haven't named the parameters of the nextcloud role well, they should all have a nextcloud_ prefix.)

BTW, again, I think a better commit message is needed.