Obsolete validation message for Monitor

[trungnn]

After the change in #64 to require type for Monitor, the following validation message is no longer valid

Kennel::Models::Base::ValidationError: foo:bar-sli-monitor type 'metric alert' is deprecated, do not set type to use the default 'query alert'

My build failed because I used to not set type and rely on Kennel to default it to 'query alert'. I think the validation error message here should be updated.