Don't create Host key in Volume if no hostPath is defined

[mcanevet]

Fixes #11

Please note that this is my very first go patch so it may have to be refactored.
I couldn't find a way to declare ecsVolume out of the if else block, and there is probably a better way to initialize the ecs.Volume structure without duplicating code.
Comments very welcome :-)

[mcanevet]

Maybe the logic of the volumes map should be changed here https://github.com/aws/amazon-ecs-cli/blob/v0.1.0/ecs-cli/modules/compose/ecs/utils/convert_task_definition.go#L47 so that the volumeName is the key and hostSourcePath the value and not the other way around.

[yinshiua]

This is merged into PR #201

[tiffanyfay]

Hey @mcanevet thanks for your PR! I cherry-picked your commit and put it in my PR with the addition of making a new volume for all empty host paths since currently your code will re-use the same volume.

[tiffanyfay]

@mcanevet Hey, I wanted to check that you're fine with your commit being merged in under the Apache 2.0 license. Thanks!

[mcanevet]

@tiffanyfj I'm fine with that.

[tiffanyfay]

@mcanevet Great, thanks !