Add ability to deploy to any install

[ryanshoover]

Adds a feature to run the deploy_site job to deploy to any install.

If you push up a branch named deploy-mywpeinstall, then it will deploy to the mywpeinstall install.

Also updates the site example to include an example usage.

To test:

1. Use any site that's set up to deploy with CircleCI
2. Edit the .circleci/config.yml file
3. Check out a new branch called deploy-nameofyourtestinstall
4. Change the orb inclusion at the top to be wpengine: ryanshoover/wpengine@dev:367496c
5. Add this blob to the bottom of the build_test_deploy workflow

               - wpengine/deploy_site:
                     name: deploy-site
                     environment: qa-site
                     requires:
                         - wpengine/lint
                         - wpengine/codeception
                     filters:
                         branches:
                             only:
                                 - /deploy-.*/
   

6. Push your branch to github
7. You should see the branch deploy to your install

[cgarza1992]

One the ssh key was added to the install for circle and the context set for the job it worked like a charm.

[ryanshoover]

@cgarza1992 We'll need a way to document the process for setting up a dev site including ssh key