providers/virtualbox: make sure sf host paths are strings
synced_folders/nfs:  use proper error classes
kernel: handle new NFS types
providers/virtualbox: fix broken NFS check
fixing error in comment #2333
providers/virtualbox: output UI
core: verify explicit sf types are usable
synced_folders/nfs: config goes here
core: fix templates syntax error
Merge branch 'pluggable-synced-folder'

This enables synced folder implementations to be completely standalone
plugins and simplifies the way synced folders are enabled for providers
by providing a standard `SyncedFolder` middleware that does everything.
This is all much better tested than previously.

The built-in synced folder types are now:

* "virtualbox" - VirtualBox shared folders

* "nfs" - NFS shared folders

These can now easily be extended
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Merge pull request #2529 from markpeek/markpeek-freebsd-nobashshell

guests/freebsd: Allow the FreeBSD plugin to install without bash [GH-2485]
Merge pull request #2521 from isaoshimizu/fix_nic_name_on_fedora

guests/fedora: fix network interface name on fedora. issue #2520
providers/virtualbox: retry suspended check [GH-2479]
Merge pull request #2487 from hatsuseno/master

hosts/gentoo: Restart NFS on Gentoo host instead of starting [GH-2487]
Update CHANGELOG
Merge pull request #2460 from jeremyfelt/patch-1

website/docs: Add CentOS 'testing' argument for install_type
Merge pull request #2448 from rimckenn/patch-1

hosts/bsd: don't set both maproot and mapall
guests/linux: increase sleep time btween NFS mount retries
Update CHANGELOG