Removes 'positive' tag from tests.

Also removes brackets where tag is a single value. This should be
finally cleanup the tags, see the following:

$ grep -r "@attr" tempest | cut -d: -f2 | sort -u
    @attr(type='gate')
    @attr(type=['negative', 'gate'])
    @attr(type='smoke')
$

Change-Id: I243800c597996c0dbcf153d66946062df6d298ab
Implements: blueprint set-gate-attribute
diff --git a/tempest/api/compute/admin/test_fixed_ips.py b/tempest/api/compute/admin/test_fixed_ips.py
index 8424709..f201cf7 100644
--- a/tempest/api/compute/admin/test_fixed_ips.py
+++ b/tempest/api/compute/admin/test_fixed_ips.py
@@ -51,7 +51,7 @@
 class FixedIPsTestJson(FixedIPsBase):
     _interface = 'json'
 
-    @attr(type=['positive', 'gate'])
+    @attr(type='gate')
     def test_list_fixed_ip_details(self):
         resp, fixed_ip = self.client.get_fixed_ip_details(self.ip)
         self.assertEqual(fixed_ip['address'], self.ip)
@@ -61,13 +61,13 @@
         self.assertRaises(exceptions.Unauthorized,
                           self.non_admin_client.get_fixed_ip_details, self.ip)
 
-    @attr(type=['positive', 'gate'])
+    @attr(type='gate')
     def test_set_reserve(self):
         body = {"reserve": "None"}
         resp, body = self.client.reserve_fixed_ip(self.ip, body)
         self.assertEqual(resp.status, 202)
 
-    @attr(type=['positive', 'gate'])
+    @attr(type='gate')
     def test_set_unreserve(self):
         body = {"unreserve": "None"}
         resp, body = self.client.reserve_fixed_ip(self.ip, body)