vconsole-setup: fix vconsole.conf vs. cmdline overriding logic

Skipping the parsing of /etc/vconsole.conf just because some values were
already assigned from the cmdline never made sense. And by the way, commit
f73141d changed the return values of parse_env_file() - it now gives 0
on success. Which means in current HEAD /etc/vconsole.conf overrides the
cmdline, which is the reverse of what's expected.

We need to parse /etc/vconsole.conf first and then let vconsole.*
overrides from cmdline take effect. The behaviour is documented in
vconsole.conf(5).

https://bugzilla.redhat.com/show_bug.cgi?id=948750