add check to create ~/.ssh/ to install-dependencies

The script breaks if ran on a fresh install without
a $HOME/.ssh/ directory. So let's make sure we have one.
Additionally, don't create (and set the mode of) one if
one already exists

Change-Id: Id7a2ef668ee31f00ecccb08a2b888f94ffbac06f
Merge "add check to create ~/.ssh/ to install-dependencies"