Missing baremetal driver API test

As a part of blueprint, this patch implements following API test -
Driver APIs -
1. Retrieve a list of drivers.

'fake' driver is created by default during ironic installation.
For this please have a look on -
https://github.com/openstack/ironic/blob/master/setup.cfg#L33

part of blueprint: missing-baremetal-api-test

Change-Id: I5abe62f2fb9aec103ce4e127a28f488ef1f342df
Merge "Missing baremetal driver API test"