support create/delete VMIface: update model

attach a network interface to vm in model
detach a network interface from vm in model

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
support create/delete VMIface: update mockmodel

attach a network interface to vm in mockmodel
detach a network interface from vm in mockmodel

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
support create/delete VMIface: update API.json

add vmifaces_create parameters verification in API.json

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
support create/delete VMIface: update testcase

update tests/test_model.py and tests/test_rest.py

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>