Install Guide: fix ssh command

* Identity file is not mykey, but ~/.ssh/id_rsa (the default).
* CirrOS demands login as user cirros, not user root.

backport: havana
(cherry picked from commit b769913e5a555a3c6b62a21cf4d4e0c5e110b45c)

Change-Id: Ifd37eaa75b3c72204dc7507c4d697af0e954901b
Merge "Install Guide: fix ssh command" into stable/havana