Fix dependency problems with APT repo handling

The first issue was that Exec[apt_get_update_for_nginx] resource is only
needed if $manage_repo == true, so it was put inside the corresponding
if block.

But even inside the if-block, Exec[apt_get_update_for_nginx] could fail
when $package_source has an invalid value and no Apt::Source[nginx] is
defined, so the respective Apt::Source[nginx], if defined, will now notify
Exec[apt_get_update_for_nginx] and Exec[apt_get_update_for_nginx] will
no longer subscribe to Apt::Source[nginx].

It would probably make sense to add a call to fail() in the default case
of $package_source.