Details: This commit is to add a new method to get target
release OS type during upgrades. The target release may be either Debian or CentOS.
By checking the existence of os_tree repo in the target release,
we can identify the OS type. If os_tree repo exists, it is Debian
Test Plan:
PASS: Added new test cases for the method.
PASS: Ran all unit test cases.
PASS: Integration test with command host-upgrade that eventually
triggers _update_pxe_config()
Task: 45887
Story: 2009303
Signed-off-by: Junfeng (Shawn) Li <junfeng.li@windriver.com>
Change-Id: I70b5b154ffa46c4390fb6fae8252073307f40997
89a7cbc2a1