exp/ssh: use ClientConfig.rand() for publickey authentication
exp/ssh: use ClientConfig.rand() for publickey authentication

Closes TODO from 5373055

R=agl
CC=golang-dev
http://codereview.appspot.com/5375081

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

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