Add exoscale provider to libcloud pack

Hi folks, little bit of a dirty hack for Christmas.
This adds the exoscale provider to the libcloud config file.
It modifies the create_vm.py script a bit so we can pass names of image and size
Fixes a typo on the json file for destroy_vm which prevented from destroying vm
Minor readme edit which I made for a test :)

Surely we could create a separate pack for exoscale and cloudstack, but it would also be nice to support everyone through the single libcloud pack.
We should throw ValueError if neither `size_id` nor `size_name` is provided. Same goes for `image_id` and `image_name.`