properly initialize compute object inside elb methods
[aws/security_group] Support mock of group from another account

It is possible to have a security group rule refer to group that is a
different account, by UserId and GroupId or GroupName. If a GroupName is
not supplied, the security group mocking implementation searches for the
security group that matches the supplied GroupId. This commit allows for
a default GroupName of nil if it is not supplied.
Merge branch 'master' of https://github.com/fog/fog into edge-gateway
fix for 1.8.7
fix EngineVersion for PendingModifiedValues (plus minor cleanup)
Merge pull request #2980 from engineyard/r3

properly initialize compute object inside elb methods
Merge pull request #2968 from dbatwa/edge-gateway

VCloud Director - Edge gateway
Merge pull request #2982 from 20goto10/feature/fix_pending_engine_version

AWS RDS - Fix EngineVersion for PendingModifiedValues (plus minor cleanup)
Merge pull request #2979 from gregburek/aws-sg-mock

[aws/security_group] Support mock of group from another account
Merge branch 'master' of https://github.com/fog/fog
WIP - Complying with VCloud Director's xml and pulling it into its own class with tests.