Add _interface attribut in BaseNetworkTest

This avoid the situation when a child class uses the _interface
attribut but it's not defined.

Other Base class (Volume, Image) seems to do a similar thing.

Patch suggested by Kirill Shileev on LP
Closes-Bug: 1360290

Change-Id: I1cdb52ab87134390cfa5f644cded38e83bf1e77c
Migrate test_server_basic_ops to tempest client

This patch migrate the server basic ops scenario tests
from official client to tempest clients.

Partially-implements: bp:tempest-client-scenarios

Change-Id: Ib9addcf1eaf0cb681ff148794e575eec4e584cf5
Migrate test_volume_boot_pattern to tempest client

Migrate the test and all shared or necessary functions from
OfficialClientTest to ScenarioTest. Create and update as well all
needed functions:

- server api schema
- servers_client

Partially implements: bp:tempest-client-scenarios

Change-Id: Ib9e70b6ce3814031d1b4b8987a24eb0992624204
Merge "Migrate test_volume_boot_pattern to tempest client"
Merge "Migrate test_server_basic_ops to tempest client"
Merge "Add _interface attribut in BaseNetworkTest"
Merge branch 'master' of https://github.com/openstack/tempest