Check for default sec-group made case insensitive

Check for default sec-group made case insensitive

Currently creating another security group with name
"default" is not allowed, however we can create
another security group with name "DEFAULT"
(or any other CASE pattern).

When trying to boot a VM without specifying
a security group it should always pick the
"default" security group. However, if another
security group of the name DEFAULT is present,
the VM gets associated with the wrong
security group (i.e DEFAULT and not default).

So, updated the code so that a security group
with name DEFAULT (or any other case pattern)
is not allowed.
Closes-Bug: #1384505

Change-Id: I3bc4e48cf723bcf74e098d7ef66df0333f6a8686