Renamed to "is_plugin_v1_enabled" and "is_plugin_v2_enabled".
Improve weave status and weave launch* err message when plugin-v2 is running
```
$ weave launch
Docker plugin "weaveworks/net-plugin" is running; Aborting.
$ weave status
<..>
Service: plugin (v2)
DriverName: weaveworks/net-plugin
```
This simplifies the following:
- plugin-v2 does not need to start "plugin" as a separate process, thus
Docker is able to properly restart plugin-v2 in a case of any child
process failing.
- No need for weaveplugin container.
plugin is enabled by default when running "weave launch". when starting
only the router, you should pass "--plugin" parameter to enable it.
The env var WEAVE_NO_PLUGIN is no more.