Add a Cassandra upgrade mechanism

[wallrj]

• Add a NodePool.Status.Version attribute which has the lowest reported version from all the pilots in the pool.
• Or nil, if the pilot failed to query its Cassandra database for its version.
• Calculate an UpdateVersion action if the desired Cassandra version is higher than the version reported for a NodePool.
• NodePools are upgraded one at a time.
• Pods within a Nodepool are upgraded one at a time, using a rolling update strategy without any partitioning.
• Upgrades are performed after Nodepools are created and after scale out.

Fixes: #257

Release note:

Navigator can now perform minor and patch upgrades of Cassandra clusters.

[jetstack-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: To fully approve this pull request, please assign additional approvers. We suggest the following additional approver: wallrj

Assign the PR to them by writing /assign @wallrj in a comment when ready.

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - **[OWNERS](https://github.com/jetstack/navigator/blob/master/OWNERS)** Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment