THe flag was unused before this point. This also sets the default state to "true", which is what I think most users would expect.

```
$ waypoint context list
    |         NAME         |  PLATFORM  |                                    SERVER ADDRESS
----+----------------------+------------+----------------------------------------------------------------------------------------
  * | is-default           | n/a        |

izaak ~ $ ~/dev/waypoint/waypoint context create -set-default=false not-default

Context "not-default" created.
$ waypoint context list
    |         NAME         |  PLATFORM  |                                    SERVER ADDRESS
----+----------------------+------------+----------------------------------------------------------------------------------------
  * | is-default           | n/a        |
    | not-default          | n/a        |
```