fix V2 auth timestamp

Amazon describes common header parameters here -
http://docs.aws.amazon.com/AWSEC2/latest/APIReference/Query-Common-Parameters.html
Timestamp should be in ISO8601 format. But datetime.isoformat() generates
string in format that not compatible with ISO8601.
This fact make botocore incompatible with Openstack implementation of Amazon
REST API (but euca2ools and boto work well with Openstack)
So I prefer to copy this from requestbuilder that used by euca2ools.