Update RESP message when failed to create flavor

The response message is unexpected(not easy to understand),if
requested resource exceeded limitation when to create flavor.
Because the error message used DB model fields but not that
used by API body. Such as following response message:
"Invalid input received: memory_mb must be <= 2147483647"
'memory_mb' is DB model field but 'ram' used in REST API

Change "memory_mb" to "ram", "root_gb" to "disk" and
"ephemeral_gb" to "ephemeral".

Change-Id: I43e2f307d21325a6b54815edf157f3d5fa1135e5
Fixes: bug #1350751
Merge "Update RESP message when failed to create flavor"