added list_headnode_images query call
fixed syntax error in tests/unit/api.py, and added cfg section for
headnode in test-common
Merge branch 'devel' into headnode_img_query

Conflicts:
	haas/test_common.py
added base-headnode to config options in test-common and changed api test to
reflect
Merge pull request #295 from lokI8/headnode_img_query

added list_headnode_images query call