Move list_hosts to resource_setup in test_hosts_negative
Move list_hosts to resource_setup in test_hosts_negative

In HostsAdminNegativeTestJSON, hosts can be get in resource_setup
to avoid getting them repeatedly in each testcase.
Besides, when a nonexistent hostname is needed, we can use the
string 'nonexistent_hostname' directly instead of creating a
random host name. Thus the whole code will look clearer.

Change-Id: Icbd10ec5cd0356763634ad4f92cad85e32d71ee6

vedujoshi/tempest,openstack/tempest,openstack/tempest,vedujoshi/tempest,cisco-openstack/tempest,Juniper/tempest,cisco-openstack/tempest,masayukig/tempest,masayukig/tempest,Juniper/tempest