Build and document 'dce upgrade' functionality

[joshmarsh]

• Add a flag to dce system deploy that specifies the version of dce to deploy.
• Default is to deploy latest version
• Add a quick start for updating dce given you have an old version deployed
• Consider how to deal with incompatibilities between different versions of the cli and dce backend

[eschwartz]

Add a flag to dce system deploy

I'd prefer this configured in the yaml file

[eschwartz]

Default behavior should be:

• Download latest version
• Write version number to yaml file, so it's pinned for the user

user can then upgrade in yaml as needed

[nathanagood]

To reduce risk, let's consider pinning the version as default behavior and instead provide a "--latest" or something flag to allow users you override the version if desired (or the user can crack open the dce.yaml file and update).