Remove "test-only" pam config options

Remove the "pam" configuration options that were only ever used for
testing purposes. Removes the test config file "backend_pam.conf"
by moving test_backend_pam.py to utilize proper configuration fixture
for identity driver. Since we no longer need the "pam.user" and
"pam.password" config options.

DocImpact

Related-Bug: #1229941
Change-Id: I3759ff7974948432900e3a73f3d87e5eed6e9828
Merge "Remove "test-only" pam config options"