Improve ssh key config

Move insert_key directive before VM create to avoid custom key create
Add comments motivating the config choices for ssh config steps
Expand user key authz logic to append key rather than just replace all
existing keys in vagrant user authorized_keys file.

Vagrantfile

@@ -3,6 +3,9 @@
 
 Vagrant.configure("2") do |config|
 
+  # don't configure host-specific keys, config will use the user's key
+  config.ssh.insert_key = false
+
   config.vm.define "ohpc" do |ohpc|
     ohpc.vm.box = "ravi89/centos7.5"
     ohpc.vm.box_version = "1"
@@ -27,9 +30,17 @@ Vagrant.configure("2") do |config|
     vb.memory = "2048"
   end
 
-  config.ssh.insert_key = false
+  # define user's key and insecure default
+  # insecure default is required for initial provisioning
   config.ssh.private_key_path = ["~/.ssh/id_rsa", "~/.vagrant.d/insecure_private_key"]
-  config.vm.provision "file", source: "~/.ssh/id_rsa.pub", destination: "~/.ssh/authorized_keys"
+  # append user's key to vagrant config to avoid overwrite of existing authorized_keys
+  # https://stackoverflow.com/a/31153912/8928529
+  config.vm.provision "ssh_pub_key", type: "shell" do |s|
+      ssh_pub_key = File.readlines("#{Dir.home}/.ssh/id_rsa.pub").first.strip
+      s.inline = <<-SHELL
+        echo #{ssh_pub_key} >> /home/vagrant/.ssh/authorized_keys
+      SHELL
+  end
 
   config.vm.provision "shell", inline: <<-SHELL
     if [ -f /vagrant/localenv.sh ]; then