Allow specifying SSH private key to _testenv

devtest_testenv.sh currently hardcodes a path for the SSH private
key, which means that CI will always create nodes with pm_password
set to null because ssh-key is set to null because the file doesn't
exist. Add a -s argument to allow specifying the private key path.

Change-Id: Icd9220e3d660d097bad6bc33090b9ef1758c5945
Merge "Allow specifying SSH private key to _testenv"