simplify ssh conn string

[smlx]

This PR makes two changes to SSH connection string generation:

1. only use -t and -p NN when necessary, instead relying on ssh defaults.
   Before:

   $ lagoon ssh --conn-string -p my-demo -e master 
   ssh -t -p 22 my-demo-master@ssh.example.com
   
   $ lagoon ssh --conn-string -p my-demo -e master -s foo
   ssh -t -p 22 my-demo-master@ssh.example.com service=foo
   

   After:

   $ go run . ssh --conn-string -p my-demo -e master 
   ssh my-demo-master@ssh.example.com
   
   $ go run . ssh --conn-string -p my-demo -e master -s foo
   ssh -t my-demo-master@ssh.example.com service=foo
   

2. stop using -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no altogether, now that ssh-core has a static host key.