[EC2] Fixed image filtering policy defaults
[EC2] Added VO -> AWS account mapping

Different VOs can now be mapped to different AWS accounts. This
is performed by the EC2 backend internally.

See 'etc/backends/ec2/files/mapfile.yaml' for details.
[EC2] Matching defaults in A2 VH and prod. config
[Gemfile] Updated deps (bug & security fixes)
[EC2] Fixes EGI-FCTF/rOCCI-server#56, waiters

Recent version of the AWS EC2 SDK implements the concept of waiters
for automatic polling of selected resources.
Merge pull request #61 from arax/ec2_fixes

[EC2] Minor fixes and pre-release improvements