New parameter os_image_name

[romachalm]

The proposed code adds a parameter os_image_name to be used instead of os_image_label. It allows to precise the name of the source image either :

• as listed in "azure vm image list", ie the hermetic [32chars id]__[human_readble_name]
• or only the last part of the name [human_readble_name] depending on what you prefer to copy paste or read.

For instance :

"os_image_name": "Ubuntu-14_04_3-LTS-amd64-server-20150805-en-us-30GB"
"os_image_name": "b39f27a8b8c64d52b05eac6a62ebad85__Ubuntu-14_04_3-LTS-amd64-server-20150805-en-us-30GB"

It should solve the issue #108 as the user can more easily find the name of the OS he is looking for, and implements the #131 .

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[paulmey]

LGTM, Travis has some unrelated issue with the Azure SDK. If you can squash the changes into a single commit, I'll merge it

[romachalm]

Done @paulmey, the four commits are now one

[paulmey]

The SDK issue is supposed to be fixed somewhere later this morning, so I'll merge this now and see if anything needs fixing... :+1: