There are some regexes used in the json-schema for complex string validation.
When the validation failed, json-schema lib didn't generate a useful error
info for the user, it shows the regex to the user. But regex is really
unreadable for normal user. This patch override the default FormatChecker to
support passed in custom error message. This required using custom format
checker instead of using 'pattern'.
For aggregates API, it enabled 'null' in the name input. As the 'format'
keyword works for all allowed types and name format check will only validate
string type, so this patch change the schema to use 'oneOf' keyword, then
the 'format' will only against on string type.
Change-Id: Ic0e608b8a18b635bfcd936f57f14c9f54e1ef8b4
Partial-Bug: #1541691
82c3afa5a4