add flag to model create flow to prevent race

- because there are 2 events fired when a user
blurs the model name input and then clicks on the
save model button it was causing a race condition
during create phase
- the race would sometimes result in 2 create calls
made for one model definition - thus the error
- there is now a simple flag set when creating the model
the first time to prevent it getting called twice

Fixes #597
Merge pull request #636 from strongloop/fix/597-error-save-model-name

add flag to model create flow to prevent race