Add optional `most_recent` parameter to `ibm_compute_image_template`

[arielsalvo]

The data source is pretty useless when using public images from IBM (among many others) that have the same name over the many different versions of the image. Adding an optional most_recent parameter allows the data source to return the highest ID number in the set instead of just the first it encounters while retaining the original behavior if unset or set to false.

This solution is the same as implemented by the analogous data source in the AWS provider

[jsutton]

@hkantare Is there something in this PR that is preventing it being merged?

[arielsalvo]

@jsutton I haven't had time to rework the solution to include the bug-fixing in softlayer-go @hkantare objected to the code I'd provided, so it is not ready to merge

[arielsalvo]

No longer relevant