Improved director JSON response parsing error message

Signed-off-by: Tom Xing <xingzhou@cn.ibm.com>
Fixed existing VM network reservation

- added integration test for network capacity
- refactored network reservation refilling
- back filled tests
- moved reserve_dynamic_networks into ResourcePool
- Improved network reservation error messages

[#76803812]

Signed-off-by: Tom Xing <xingzhou@cn.ibm.com>