only use the configured "IdentityFile"

from `ssh_config`:

-- IdentitiesOnly
Specifies  that ssh(1) should only use the authentication identity files
configured in the ssh_config files, even if ssh-agent(1) or a
PKCS11Provider offers  more identities. The argument  to  this  keyword
must be `yes` or `no`. This option is intended for situations where
ssh-agent offers many different identities. The default is `no`.

This can lead to incorrect behaviour when i.e talking to github.com
when another identity exists in the environment (i.e. ssh-agent). As
*all* keys that reside on github.com are able to ssh there, the first
one will win, but won't necessarily have proper access to the
repository.

set `IdentitiesOnly` to `yes` by default.

@s0enke PHAL