This allows a plugin to be upgraded without requiring to
uninstall/reinstall a plugin.
Since plugin resources (e.g. volumes) are tied to a plugin ID, this is
important to ensure resources aren't lost.
The plugin must be disabled while upgrading (errors out if enabled).
This does not add any convenience flags for automatically
disabling/re-enabling the plugin during before/after upgrade.
Since an upgrade may change requested permissions, the user is required
to accept permissions just like `docker plugin install`.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
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