Use oslo.utils

Modules `strutils`, `timeutils` and `network_utils` from common code are
graduated in `oslo.utils`, so we can:
 1. remove `novaclient.openstack.common.network_utils` and use
    `oslo.utils.netutils` instead.
 2. use `oslo.utils.encodeutils` and `oslo.utils.strutils` instead of
    `novaclient.openstack.common.strutils`.
 3. use `oslo.utils.timeutils` instead of
    `novaclient.openstack.common.timeutils`.

Additional information:
 - modules `importutils`, `strutils` and `timeutils` from
   `novaclient.openstack.common` cannot be removed, because:
   - importutils is used by apiclient and jsonutils;
   - strutils is used by apiclient, cliutils and jsonutils;
   - timeutils is used by jsonutils
 - additional check for `safe_encode` in Py3 is required, since
   If91a866d864a22d28a352152beff4c7406a27b7b was merged.

Change-Id: Ib8d79d9c85af4916e87a76a1a67a13488ddaa111
Merge "Use oslo.utils"