Fix unstability in test_subnet_details

when the image has some dns preconfigured in /etc/resolv.conf,
for example:

resolv.conf
nameserver 192.168.122.1

the test failed due to:
testtools.matchers._impl.MismatchError:
{'1.2.3.4'} != {'1.2.3.4', '192.168.122.1'}: Looking for servers: ['1.2.3.4'].
Retrieved DNS nameservers: ['192.168.122.1', '1.2.3.4'] From host: 10.0.0.234

DHCP renewal is forced in the server that will cleanup the file
/etc/resolv.conf from any other preconfigured DNS entry. When
the DHCP server pushes the new configuration, the Neutron network
DNS entries are applied

Closes-Bug: #1914229
Change-Id: I9bd8b47642891a1fe42c48a0b9fe50cf9bc4e03b
1 file changed