template integrity verification: verify storagepool, update API.md

Sometimes, user create a template, storagepool will change later.
So users can not create a vm from this template successfully.
It is necessary to check storagepool of a template.

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

update mockmodel and model

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

update test_rest.py
do not update test_model.py.
for commit c4c093e does not allow to delete pool kimchi-test-pool
as it is associated with some templates

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