Change pumphouse/cmds/migration.py to standard config format

Now 'pumphouse' command works with the following format of
configuration file:

  CLOUDS:
    source:
      endpoint:
	auth_url: http://endpoint.example.com:5000/v2.0
	username: admin
	password: password
	tenant:	  admin
      identity:
	connection: mysql+connector://root:password@db.example.com/keystone
Single configuration file

Remove doc/samples/api-config.yaml. Single configuration file is
doc/samples/config.yaml.

Update SCRIPTS.README.md document.
Single configuration file