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.
I prefer to copy this from requestbuilder that used by euca2ools.
add test for #308
Merge branch 'Andrey-mp-develop' into develop

* Andrey-mp-develop:
  add test for #308
  fix V2 auth timestamp