vSphere support for binary deploys

Aside form working with binary deploys, this PR changes the following two items (vSphere only):

* The reactor on the salt master is configured to run `state.highstate` on *every* node whenever any minion starts. This solves a timing issue introduced with the binary deploys; the master can be done with provisioning much sooner than the minions, meaning the apiserver configuration won't be seeded with the right list of minion IPs. This is solved by running `state.highstate` on minion starts, which causes the apiserver configuration to be rewritten and the daemon to be cycled every time a new minion starts.
* To verify the minions have started from kube-up, the script now polls `http://<ip>:10250/healthz`. The check that is executed from the GCE scripts depends the minions having completed provisioning before the master becomes available. This allows for the case where docker is not yet provisioned, while the master is already responding to requests, which in turn can cause the sanity check to fail.

cc #1732
I reworked this to use ssh-agent. No more looking around for the right keys. It now assumes that either 1) you have ssh-agent running somewhere and it has one or more available identities, or 2) you have a key at a default location with or without a passphrase. In the first case, the script doesn't do anything and just passes through `SSH_AUTH_SOCK` (it uses `ssh-add -L` to get a list of public keys associated with the available identities). In the second case, it starts ssh-agent and runs ssh-add without arguments to add a default key. If neither of these paths work, the user will have to set up ssh-agent manually.