`sls plugin install` failing
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# This is a Bug Report

## Description

* What went wrong?
I wanted to install a serverless plugin through the CLI ` $ sls plugin install serverless-express`  but was met with an error `  Serverless command "plugin install serverless-express" not found. Did you mean "uninstall"? Run "serverless help" for a list of all available commands.`
* What did you expect should have happened?
I expected based on the following that I could use the `sls` command line to install a plugin. I normally just install by adding it to my `serverless.yml` and using, but I gave this a try for the first time and it wasn't as I expected
```bash
tyler:~/environment/sls-starters/sls-express (master) $ sls help | grep plugin
* Pass "--verbose" to this command to get in-depth plugin info
install ....................... Install a Serverless service from GitHub or a plugin from the Serverless registry
plugin ........................ Plugin management for Serverless
plugin install ................ Install and add a plugin to your service
plugin uninstall .............. Uninstall and remove a plugin from your service
plugin list ................... Lists all available plugins
plugin search ................. Search for plugins
tyler:~/environment/sls-starters/sls-express (master) $ sls plugin list | grep serverless-express
serverless-express - Making express app development compatible with serverless framework.
tyler:~/environment/sls-starters/sls-express (master) $ sls plugin install serverless-express

  Serverless Error ---------------------------------------

  Serverless command "plugin install serverless-express" not found. Did you mean "uninstall"? Run "serverless help" for a list of all available commands.

  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Issues:        forum.serverless.com
 
  Your Environment Information ---------------------------
     OS:                     linux
     Node Version:           8.10.0
     Serverless Version:     1.41.0
 
tyler:~/environment/sls-starters/sls-express (master) $ sls plugin install help                                                                                            
 
  Serverless Error ---------------------------------------
 
  Serverless command "plugin install help" not found. Did you mean "uninstall"? Run "serverless help" for a list of all available commands.
 
  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Issues:        forum.serverless.com
 
  Your Environment Information ---------------------------
     OS:                     linux
     Node Version:           8.10.0
     Serverless Version:     1.41.0
```
 
* What was the config you used?
Basic starter from `$ serverless create --template aws-nodejs --path my-service` no custom config. `serverless.yml` here 
```yml
service: sls-express 
provider:
  name: aws
  runtime: nodejs8.10
functions:
  hello:
    handler: handler.hello
```

* What stacktrace or error message from your provider did you see?
```bash
  Serverless Error ---------------------------------------

  Serverless command "plugin install serverless-express" not found. Did you mean "uninstall"? Run "serverless help" for a list of all available commands.

  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Issues:        forum.serverless.com
 
  Your Environment Information ---------------------------
     OS:                     linux
     Node Version:           8.10.0
     Serverless Version:     1.41.0
 
```
Similar or dependent issues:
* #4258

## Additional Data

* ***Serverless Framework Version you're using***:
1.41.0
* ***Operating System***:
Amazon Linux inside an EC2 using Cloud9
* ***Stack Trace***:
N/A
* ***Provider Error messages***:
```bash

  Serverless Error ---------------------------------------

  Serverless command "plugin install serverless-express" not found. Did you mean "uninstall"? Run "serverless help" for a list of all available commands.

  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Issues:        forum.serverless.com
 
  Your Environment Information ---------------------------
     OS:                     linux
     Node Version:           8.10.0
     Serverless Version:     1.41.0

```