Only use `IdentityFile` for ssh authentication

[simcod]

Description

Recently, connecting to the leaf switches was not possible, due to authentication failures. As a result metal-core and bgp could not be restarted, which is necessary to add the default route (see #262).

Adding IdentitiesOnly fixed this, to only use the ssh key specified in the files/ssh/config.

[mwindower]

clashes with dell_sonic because there we use password based authentication in the virtual environment atm