This commit modifies containerboot to call the local api endpoint to
disconnect from the control plane upon shutdown. This functionality
is enabled using a new environment variable:
`EXPERIMENTAL_DISCONNECT_ON_SHUTDOWN`
When using an instance as a HA app connector or subnet router this
environment variable should be set to give connected clients enough
time to update their netmap and fail over to the next active instance.
This commit also includes some minor spelling/grammar fixes.
Updates #14020
Signed-off-by: David Bond <davidsbond93@gmail.com>
7a8976b312