Force overwrite of salt configs and keys.

This will tell the salt-bootstrap script to overwrite the master/minion
configs and/or keys each time provision is called. #3524 #3176
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Merge pull request #3536 from shadowfax-chc/force-overwrite-salt-configs

provisioners/salt: force overwrite of salt configs and keys.
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website/www: add latest-version.json to website
commands/version: check latest version
core: add Vagrant.latest_version
Merge pull request #3540 from mitchellh/f-version-command

vagrant version command
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Revert "core: can't add boxes from network shares [GH-3279]"

This reverts commit 41636a8a9cefa68d18e641efee9fc410ed1dd1d4.

Conflicts:
	CHANGELOG.md
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core: more liberally release port leases
core: delete environment lock files
core: clean up the lock cleanup code, tests
core: Box#in_use? as an API