'Returns the cluster tag name from a security group name that starts
with static.SECURITY_GROUP_PREFIX
Example:
sg = \'@sc-mycluster\'
print get_tag_from_sg(sg)
mycluster'

def get_tag_from_sg(self, sg):

    regex = re.compile(('^' + (static.SECURITY_GROUP_TEMPLATE % '(.*)')))
    match = regex.match(sg)
    tag = None
    if match:
        tag = match.groups()[0]
    if (not tag):
        raise ValueError(('Invalid    cluster    group    name:    %s' % sg))
    return tag
