labs: iniset_sudo: use privileges for reading, too

As noted in the commit message when the function was introduced,
iniset_sudo uses root privileges only for writing.

Turns out that this minimal set of root privilege use is not sufficient
for all client script needs. For instance, /etc/keystone permissions
deny access to anyone but the keystone user.

This patch allows iniset_sudo to work even if the regular user has no
permission to read the file.

Implements: blueprint openstack-training-labs
Change-Id: I85b15703a59befa531cc99136a282bedc137cde1
Merge "labs: iniset_sudo: use privileges for reading, too"