nfs_apply :: exportfs sometimes leaves exports behind

On my machine i had a case where /etc/exports was updated but the old
exports were still there. This was leading to
"exportfs: duplicated export entries" and eventually leading to nfs
being not available for the box.

changing the command exportfs -r to exportfs -ar seems to address this
issue.

Signed-off-by: BlackEagle <ike.devolder@gmail.com>
core: Within a Bundler env, don't manage Bundler
core: For dev, temporarily use branch of Childprocess, fix Win issue
providers/virtualbox: network config uses proper format, tests [GH-2854]
Merge pull request #2851 from BlackIkeEagle/reload-nfs-exports

guests/linux: use exportfs -a flag to avoid stale exports
core: clarify some logic for colors
core: cleaner output when running outside installer
Update dep of ChildProcess to 0.4.0

This fixes some critical bugs with Windows
hosts/gentoo: fix nonstandard path to systemctl command
Merge pull request #2858 from timgus/gentoo_systemctl

hosts/gentoo: fix nonstandard path to systemctl command
Update CHANGELOG
hosts/gentoo: more robust check for systemctl path on Gentoo
guests/coreos: restart etcd after configuring networks [GH-2852]