[system-tests] fuel-devops raises an exception while clearing previous environment if some objects don't exist in libvirt

Affected: fuel-devops branch release/2.9 , after the tag 2.9.19.

If some libvirt objects (networks, nodes, network filters) were removed manually, then dos.py fails to erase such environment.

fuel-devops shouldn't fail to erase environment if some libvirt objects are missing.

$ dos.py erase demo-test-new30

libvirt: Network Driver error : Network not found: no network with matching uuid
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
libvirt: Network Filter Driver error : Network filter not found: no nwfilter with matching name 'demo-test-new30_management'
Traceback (most recent call last):
  File "/home/dis/devops-venv-2.9/bin/dos.py", line 22, in <module>
    main()
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/shell.py", line 592, in main
    Shell(args).execute()
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/shell.py", line 49, in execute
    self.commands.get(self.params.command)(self)
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/shell.py", line 85, in do_erase
    self.env.erase()
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/models/environment.py", line 136, in erase
    network.erase()
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/models/network.py", line 128, in erase
    self.remove(verbose=False)
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/models/network.py", line 136, in remove
    self.undefine_filter()
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/models/network.py", line 115, in undefine_filter
    self.driver.network_filter_undefine(self)
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/helpers/retry.py", line 27, in wrapper
    return func(*args, **kwargs)
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/devops/driver/libvirt/libvirt_driver.py", line 312, in network_filter_undefine
    nwfilter = self.conn.nwfilterLookupByName(network_name)
  File "/home/dis/devops-venv-2.9/local/lib/python2.7/site-packages/libvirt.py", line 4240, in nwfilterLookupByName
    if ret is None:raise libvirtError('virNWFilterLookupByName() failed', conn=self)
libvirt.libvirtError: Network filter not found: no nwfilter with matching name 'demo-test-new30_management'