[Fix] DNS: Build next page URL from marker when API omits the "next" link

[artem-lifshits]

What this PR does / why we need it

Fixes #952

Acceptance tests

=== RUN   TestZonesList
--- PASS: TestZonesList (2.39s)
=== RUN   TestZonesCRUD
    zones_test.go:33: Attempting to create DNS public zone
    zones_test.go:42: Created DNS public zone: 2c9c2de79d7175d8019f2733068a524c
    tools.go:75: {
          "hostname": "ns1.open-telekom-cloud.com.",
          "priority": 1
        }
    tools.go:75: {
          "hostname": "ns2.open-telekom-cloud.com.",
          "priority": 2
        }
    zones_test.go:59: Attempting to update DNS public zone: 2c9c2de79d7175d8019f2733068a524c
    zones_test.go:66: Updated DNS public zone
    zones_test.go:52: Attempting to delete DNS public zone: 2c9c2de79d7175d8019f2733068a524c
    zones_test.go:55: Deleted DNS public zone: 2c9c2de79d7175d8019f2733068a524c
--- PASS: TestZonesCRUD (1.39s)
PASS

Process finished with the exit code 0