Add support for creating instances with a default network id

Nova api refuses to create a new instance when more than one network available.

We must specify a network id to avoid failure of creating test servers.
Add tests for volume attachments of servers
Fix some typo in test_verify_extended_attributes
Complement for some changes in xml interface
Change the device name of device_name_mapping_v2 to avoid issues
use relative path instead of absolute path when locating 'addresses'

There are two different xml structures sharing this translating-function:

<server ...>
...
</server>

and

<servers ...>
    <server ...>
    ...
    </server>
</servers>

It is inappropriate to locate the attribute 'addresses' using the absolute path.
Fix 'os-extended-volumes:volume_attached' mismatch between json and xml interface
add test for image extended attributes
add test for list images with parameter 'name', 'owner' and 'tag'
complement tests of filtering images in detail with api v1
implement test_set_membership in image service api v1
add some tests for listing images filter by marker
code cleanup
add test for creating server with boot volume
Merge pull request #7 from oubeichen/patch

implement some tests in compute and add some attribute in configure file