Nova cells use '!', '.' and '@' for routing the instance in cells.
So those special characters can't include in the cell name.
This patch adds '@' validation for v2 API, and adds '!', '.' and '@'
for v2.1 API.
Change-Id: I5cb704a44c2abfb0bfb5b42d4aa8fefe35cc1523
Partial-Bug: #1491511