hosts/bsd: sort NFS exports to avoid false validation errors [GH-2927]

With a synced folder configuration like so:

 synced_folder ".", "/vagrant", :nfs => true
 synced_folder "#{ENV['HOME']}/mirror", "/mirror", :nfs => true
 synced_folder ENV['HOME'], "/home/#{ENV['USER']}", :nfs => true

on OSX, vagrant writes two overlapping exports to /etc/exports
which then fail the export check.

Iterating through the list of folders lexically builds a correct, single
exports entry.

Signed-off-by: Teemu Matilainen <teemu.matilainen@iki.fi>
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