Implement integrity verification: verify template integrity, update API.md

Sometimes, user create a template, but networks, cdrom, disks or
storagepool will change later.
So users can not create a vm from this template successfully.
It is necessary to check some paramenters of template.

This patch will check the follow paramenters of template.
networks: check networks exists.
cdrom: check cdrom is available.
disks: check the volume is available.

This patch does not check the storagepool exists. waiting for royce's
disks patch.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
add a new method to get iso info for VMTemplate class

move the related code about get iso info to a new function.
Then template integrity can make use of it.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Implement integrity verification: verify template integrity in backend

update controller, mockmodel and model

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Implement integrity verification: update test case

update test_model.py and test_rest.py

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