Make passphrase file optional in protogeni test scripts.
Make passphrase file optional in protogeni test scripts.

Passphrase file is used if it exists and is readable. If passphrase
file does not exist or is not readable, invoke standard M2Crypto
passphrase callback to prompt user for passphrase.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome