As disussed in project plan, we decided to adopt etcd to store
container data which is faster and more flexible than mysql.
This commit enable etcd DB backend testing pipeline in Zun
and adjust the code correspondingly:
1. Set the value of 'db_type' to 'etcd' when runing etcd
pipeline: 'gate-zun-devstack-dsvm-docker-etcd-nv'.
2. Drop the 'get_XXX_by_id' methods from etcd API since 'id'
attribute makes no sense in etcd data model.
3. Adjust DB related test cases.
4. Make etcd write request thead-safe.
After the etcd related code been well tested, we'll change the
default DB type from 'mysql' to 'etcd'.
Part of blueprint etcd-db-driver
Change-Id: I06ac9b719ce93898cd4f67f1c7bdc2e0803e086e
96bd39afad