Thus far, the security group name was checked with Django's validate_slug.
Consequently, some characters like spaces (bug #1224576) and @ (bug #1233501) were forbidden.
This fix removes validate_slug and provides a less restrictive validation.
Now, all printable ASCII characters -- i.e. from 0x20 to 0x7E -- are valid.
Note that diacritics are disallowed.
On top of that the length is now checked and limited to 255 characters in accordance with Nova.
Security group forms have been factorized in one abstract base class: GroupBase.
CreateGroup and UpdateGroup subclass GroupBase in order to be more DRY.
Change-Id: Ifc8e5f75419a73a353b2138641773d36138ecea8
Closes-Bug: #1224576
Closes-Bug: #1233501
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