image: set fs label = / on root vol (s3->ebs)
awsutils: clear account attr cache if region changes

Updated EasyEC2's support_platforms and default_vpc properties to wipe
the cache and refetch the account attributes if the current region
doesn't match the region from the last fetch.
spothistory: use classic or vpc prices based on default vpc

Updated EasyEC2.get_spot_history to either show VPC or EC2-classic spot
prices based on whether or not the user has a default vpc. If the user
does have a default VPC then the default spot prices are VPC otherwise
it defaults to EC2-classic prices. Also added vpc=False and
classic=False kwargs to EasyEC2.get_spot_history which forces the
function to return prices for a given platform regardless of whether or
not the user has a default VPC.
spothistory: add --vpc and --classic options

Added these options to allow the user to override the spot price
platform autodetection that occurs based on whether or not the user has
a default VPC. These options are mutually exclusive.
Merge branch 'spot-history-improvements' into develop

closes gh-343