This commit fixes issues #222

Added this feature.

To use, just edit the cloud section of Config.groovy like so:
{code}
cloud {
	accountName='prod'
	publicResourceAccounts=[]
	securityGroupBlacklist=['default']
}
{code}

By default, it does not write that line to the file, and blacklists no security groups. This is because having the default group missing from the list can be surprising to users, and most asgard users will not want to filter out any groups.