Fix cluster upgrading to default version always

[martina-if]

The upgrade/update cluster commands where always trying to upgrade to the default version when
one was not specified by the user.

eksctl upgrade cluster  martina-test-1-13
[ℹ]  eksctl version 0.24.0-dev+2c97a450.2020-07-10T08:09:08Z
[ℹ]  using region eu-north-1
Error: upgrading more than one version at a time is not supported. Found upgrade from "1.13" to "1.16". Please upgrade to "1.14" first

eksctl upgrade cluster martina-test-1-13
[ℹ]  eksctl version 0.24.0-dev+19f9b371.2020-07-10T09:30:41Z
[ℹ]  using region eu-north-1
[ℹ]  (plan) would upgrade cluster "martina-test-1-13" control plane from current version "1.13" to "1.14"
[ℹ]  re-building cluster stack "eksctl-martina-test-1-13-cluster"
[✔]  all resources in cluster stack "eksctl-martina-test-1-13-cluster" are up-to-date
[ℹ]  checking security group configuration for all nodegroups
[ℹ]  all nodegroups have up-to-date configuration
[!]  no changes were applied, run again with '--approve' to apply the changes

The problem is that when we create a new ClusterConfig object we set the version to the default version. Then this
value is used as a default for the --version flag.

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Added labels for change area (e.g. area/nodegroup), target version (e.g. version/0.12.0) and kind (e.g. kind/improvement)
• Make sure the title of the PR is a good description that can go into the release notes

[michaelbeaumont]

Argh I don't like that there's a massive list of "Check PR labels" statuses when looking at the commit statuses and that they don't overwrite each other and there's an X because the first one failed 😒