Add Linux Mint detection

* Linux Mint is a popular distribution based on Ubuntu
* When creating a VM based on Linux Mint, it cannot detect
that it is based on Ubuntu and it is detected as just linux
and configure_networks fails
* https://github.com/mitchellh/vagrant/issues/3647
Up version for dev
Merge pull request #3648 from chrisvire/fix/master/add_mint_guest

Add Linux Mint detection
guests/mint: don't need to inherit from ubuntu class
Update CHANGELOG
providers/virtualbox: check for provider config nilness
Revert "Merge pull request #3643 from cammoraton/rhel7-plugin-fixes"

This reverts commit 919d0b9f4b77cd9f2d2bd050fc2db5f00eeebabd, reversing
changes made to 924c47feb39354cf6ddb008a23a425c404c61446.
Update CHANGELOG
core: no active machines if there is no vagrant env
synced_folders/rsync: mkdir before rsync
providers/virtualbox: package --base should use vboxsf
Fix error reporting bug

There's no "command" variable; it should be "cmd". This bug causes it to print an unhelpful exception if the Docker command fails.
Merge pull request #3654 from mithaler/patch-1

providers/docker: fix Docker error reporting bug
Update CHANGELOG.md
commands/reload: if --provision is present, force provision [GH-3657]
guests/*: setup rsync_pre properly
guests/windows: nicer error if winrm not in use to configure networks [GH-3651]
providers/docker: can set default provider to docker [GH-3662]
core: public_send vs send when merging configs

This fixes the problem when config keys collide with Kernel/Object
methods (private). An example is `exec` which is used in vagrant-exec
plugin.

Compare:

> old.send :exec
ArgumentError: wrong number of arguments (0 for 1+)
> old.public_send :exec
=> #<Vagrant::Config::V2::DummyConfig:0x007fe212cc05c8>
providers/docker: Rename test files for consistency