While YAML does have inbuilt support for anchors to greatly reduce
duplicated sections, anchors have no support for merging values. For
diskimages, this can result in a lot of duplicated values for each
image which you can not otherwise avoid.
This provides two new values for diskimages; a "parent" and
"abstract".
Specifying a parent means you inherit all the configuration values
from that image. Anything specified within the child image overwrites
the parent values as you would expect; caveats, as described in the
documentation, are that the elements field appends and the env-vars
field has update() semantics.
An "abstract" diskimage is not instantiated into a real image, it is
only used for configuration inheritance. This way you can make a
abstrat "base" image with common values and inherit that everywhere
without having to worry about bringing in values you don't want.
You can also chain parents together and the inheritance flows through.
Documentation is updated, and several tests are added to ensure the
correct parenting, merging and override behaviour of the new values.
Change-Id: I170016ef7d8443b9830912b9b0667370e6afcde7
b5b20b6e2c