Fix test_positive_mismatched_satellite_fqdn

[rmynar]

Test seemed to randomly fail on assertion at https://github.com/SatelliteQE/robottelo/blob/952b960381ce5817ffdd5598c3d9fc4ef8b9fff0/tests/foreman/destructive/test_installer.py#L109

After some investigation I found out that the domain is being generated randomly and for some domains (e.g. test.com, test.biz) the output of hostname --fqdn returns FQDN, not just "shortname" as we expected.

Removing nameservers from /etc/resolv.conf solves the issue.

> hostnamectl set-hostname testname # set test hostname
> echo "search example.info" > /etc/resolv.conf # set search domain to example.info
> hostname --fqdn # hostname prints only hostname without domain name
testname
> sed -i "s/^search.*/search test.biz/" /etc/resolv.conf # set search domain to test.biz
> hostname --fqdn # for some reason the fqdn is printed
testname.test.biz
> sed -i "/nameserver.*/d" /etc/resolv.conf # remove nameservers
> hostname --fqdn # only hostname without domain name is printed
testname