[yoga] nova-api-bootstrap fails due to missing MySQLdb

[Crazik]

Env:
Kolla-ansible 14.8.0 in a process of upgrading from Xena to Yoga
Images used: ubuntu-source from quay.io

In the process of doing upgrade, I've met this case:
Task "Running Nova API bootstrap container" fails with final error:

Error: No module named 'MySQLdb'

image used: quay.io/openstack.kolla/ubuntu-source-nova-api:yoga
hash: 2c3ada1c3d86

As pip3 module installation fails (requirements missing), solution is to install python3-mysqldb with apt.

steps to fix:
1. docker run -it --name tmp quay.io/openstack.kolla/ubuntu-source-nova-api:yoga bash
2. apt-get update && apt-get install python3-mysqldb
3. docker commit <container_id> TMP_TAG
4. docker tag TMP_TAG quay.io/openstack.kolla/ubuntu-source-nova-api:yoga
5. re-run kolla-ansible process.

note: that may happen only if there is a cell with "mysql://" connection scheme.