core: Vagrant.has_plugin? tries to match gem name first

Search primary from the list of gem names which have been loaded by
`Vagrant.require_plugin`. Fall back to matching registered plugin names.
Merge pull request #2617 from tmatilai/has_plugin_by_gem_name

`Vagrant.has_plugin?` should use the gem name
Update CHANGELOG
website/docs: Fix some typos in plugin docs
Fixed typo in boxes/base.html.md
Merge pull request #2706 from Ninir/patch-1

website/docs: Fixed typo in boxes/base.html.md
Set default value to avoid a prompt
Merge pull request #2707 from tobiasbaehr/patch-1

guests/debian: Docker: Set default value to avoid a prompt (lxc/directory)