Remove set_mac_address from old remote_client

set_mac_address() was called from test_network_basic_ops only and
it is not necessary to keep it as common method in Tempest.
Then this patch moves the implementation into test_network_basic_ops
and removes set_mac_address() from old remote_client.
In addition, this patch merges 3 exec_command() calls into a single
call for avoiding ssh re-connections.

Change-Id: I60b86a143e3987b911fa070754879bd002128120
2 files changed