ec2-api/doc/source/install/configuration.rst

To configure OpenStack for EC2 API service add to /etc/ec2api/ec2api.conf:

[DEFAULT]
external_network = public
ec2_port = 8788
ec2api_listen_port = 8788
keystone_ec2_tokens_url = http://192.168.56.101/identity/v3/ec2tokens
api_paste_config = /etc/ec2api/api-paste.ini
disable_ec2_classic = True

In the [keystone_authtoken] section, configure Identity service access.

[keystone_authtoken]
project_domain_name = Default
project_name = service
user_domain_name = Default
password = password
username = ec2api
auth_type = password

Also you need to configure database connection:

[database]
connection = mysql+pymysql://root:password@127.0.0.1/ec2api?charset=utf8

and you need to configure oslo_concurrency lock_path:

[oslo_concurrency]
lock_path = /path/to/oslo_concurrency_lock_dir

and cache if you want to use it.

[cache]
enabled = True

You can look for other configuration options in the Configuration Reference