title: netplan not working after change default python
body: <p>in ubuntu 18.04, when i change default python from python 3.6 to other version by this command:</p>
  <pre><code>sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1
  sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.7 1
  </code></pre>
  <p>or when i remove python 3.6 and install other version <code>netplan apply</code> not working and result this error:</p>
  <pre><code>  File &quot;/usr/sbin/netplan&quot;, line 20, in &lt;module&gt;
      from netplan import Netplan
    File &quot;/usr/share/netplan/netplan/__init__.py&quot;, line 18, in &lt;module&gt;
      from netplan.cli.core import Netplan
    File &quot;/usr/share/netplan/netplan/cli/core.py&quot;, line 24, in &lt;module&gt;
      import netplan.cli.utils as utils
    File &quot;/usr/share/netplan/netplan/cli/utils.py&quot;, line 25, in &lt;module&gt;
      import netifaces
  ModuleNotFoundError: No module named 'netifaces'
  </code></pre>
  <p>and command <code>pip install netifaces</code> has some errors.</p>