Add api extension to get and reset password

Password is stored in system_instance_metadata as four items with the
keys 'password_0' through 'password_3'. The extension adds a resource
endpoint with two methods:

GET v2/servers/<uuid>/os-server-password  # get_password
DELETE v2/servers/<uuid>/os-server-password # reset_password

get_password retuns:

JSON: {"password": "xxx"}
XML: <?xml version='1.0' encoding='UTF-8'?><password>xxx</password>

Note that this is intended to be an encrypted password set by the
guest. Includes api tests for json and xml. Also includes api samples.

Part of blueprint get-password

Change-Id: I6c03f9c3bc8a2c70403bbb3e57917ab9522d75bd
Merge "Add api extension to get and reset password"