Follow API-SIG guidelines on microversions

[stbenjam]

See here: http://specs.openstack.org/openstack/api-sig/guidelines/sdk-exposing-microversions.html

microversion should probably not be exposed to end users.

[stbenjam]

Will need this in gophercloud first: gophercloud/gophercloud#1577

[hardys]

@stbenjam gophercloud/gophercloud#1577 landed, what work remains to close out this issue?

[stbenjam]

That was the easiest part, now we'll have to remove the microversion field from the provider and then dynamically determine the minimum version to use based on the features the user is requesting. I haven't thought a lot about the best way to do this yet, but I don't think it's super high priority in the short term.