vm ticket in backend: update API.md

A ticket is the credential to access VM.
Only who get the ticket can access a VM.

test this patch set:
set the ticket
$ sudo curl -k -u <user>:<password> -H "Content-Type: application/json" -H  \
"Accept: application/json" https://localhost:8001/vms/test-vm-8/ \
-X PUT -d '{"ticket": {"passwd": "abcd"}}'

get the ticket
$ sudo curl -k -u <user>:<password> -H "Content-Type: application/json" -H \
"Accept: application/json" https://localhost:8001/vms/test-vm-8/

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
vm ticket in backend: update controller and API.json

verify parameters when update the ticket of a VM.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
vm ticket in backend: update model

set ticket as attribute of vm info.

vm lookup will get ticket.

API user should check the expire of ticket.

vm update method can edit ticket.

when set a ticket, it will take effect on both persistent and live
configure.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
vm ticket in backend: update mockmodel

Support lookup and update ticket for vm.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
vm ticket in backend: update test case

update test_model, test_mockmodel and test_rest

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