config version API support: add a method to get kimchi version

add a method to get kimchi version in config.py

No matter kimchi package installed or not, we will always
get version and release.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
config version API support in backend.

update API.md, mockmodel, model and API.md

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