If one is seeing this on a Ubuntu / Debian box during start of keystone after migrating DB from sqlite to MySQL, simply installing the associated python libraries would fix the issue.
root@keystone:~/openstack-keystone-79a9fde# ERROR: Unable to load keystone-legacy-auth from configuration file /etc/keystone/keystone.conf.
Got: ImportError('No module named MySQLdb',)
root@keystone:~/openstack-keystone-79a9fde# apt-get install python-mysqldb
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
The following NEW packages will be installed:
0 upgraded, 2 newly installed, 0 to remove and 58 not upgraded.
Need to get 109 kB of archives.
After this operation, 578 kB of additional disk space will be used.
Do you want to continue [Y/n]? y
Get:1 http://us.archive.ubuntu.com/ubuntu/ oneiric/main python-support all 1.0.13ubuntu1 [26.6 kB]
Get:2 http://us.archive.ubuntu.com/ubuntu/ oneiric/main python-mysqldb amd64 1.2.3-0ubuntu1 [82.5 kB]
Fetched 109 kB in 0s (184 kB/s)
Selecting previously deselected package python-support.
(Reading database ... 55750 files and directories currently installed.)
Unpacking python-support (from .../python-support_1.0.13ubuntu1_all.deb) ...
Selecting previously deselected package python-mysqldb.
Unpacking python-mysqldb (from .../python-mysqldb_1.2.3-0ubuntu1_amd64.deb) ...
Processing triggers for man-db ...
Setting up python-support (1.0.13ubuntu1) ...
Setting up python-mysqldb (1.2.3-0ubuntu1) ...
Processing triggers for python-support ...
root@keystone:~/openstack-keystone-79a9fde# keystone &
root@keystone:~/openstack-keystone-79a9fde# Starting the RAX-KEY extension
Starting the Legacy Authentication component
Service API listening on 0.0.0.0:5000
Admin API listening on 0.0.0.0:35357