Manage image ID's for node profile

Kernel and ramdisk image ID's are stored for each node profile
in nova keys and should be managed via UI.

Only image ID is displayed in node profile list. Though
somewhat unfriendly, this is better for now than making
2 calls to Glance for every row. Separate node details page
will be introduced in the next patch to fix this issue.

Also this patch introduce abstraction over flavor to ease
writing code specific to node profiles.

Change-Id: Ie76651536e2d8b90e14c9000da0226743db91e88