When creating a volume, the scheduler hints can be supplied as strings.
The "same_host" and "different_host" hints can also be supplied as a
list if affinity/anti-affinity to multiple volumes is requested [0]
The previously-used `KeyValueAction` only supplies strings as values - the
last one if multiple --hint contain the same key. An alternative already
used in `CreateServer` would be `KeyValueAppendAction`, but only a subset of
the scheduler hints accept lists, so we cannot use that in general.
Therefore, we create `KeyValueHintAction`. It contains both a
`KeyValueAction` and a `KeyValueAppendAction` object and calls the
appropriate action based on the beginning of the given values as defined
in `APPEND_KEYS`.
[0] d96b705774/cinder/api/schemas/scheduler_hints.py (L31-L65)
Change-Id: Ida7f4662dc9fea24510758541fd4f2622b73bf31
60a0e379a4