REST API: Fix needed capabilities for update and install plugins.

Use more specific `update_plugins` and `install_plugins`
API endpoints: Remove duplicate code
Plugin Modify Endpoint

Fixing a few error that was causing the endpoint to fail to recognize a plugin in the url path.
Plugin Modify Endpoint

Fixing a few error that was causing the endpoint to fail to recognize a plugin in the url path.
Merge pull request #1310 from Automattic/update/plugins-install-update-capabilities

Update/plugins install update capabilities