Heat Stack name is missing form validation.

When creating a stack using Horizon, the form is missing name
validation, so a generic error is shown whenever the name
does not follow the regex used by Heat.
([a-zA-Z][a-zA-Z0-9_.-]*)
Add server side validation and the right error message.

Change-Id: I7a639bcf96b7c74760123b6106aab7d9311f5e2f
Closes-Bug: #1254212
Merge "Heat Stack name is missing form validation."