Add password option to cred store

Add password option to cred store

Add an option for initial creds acquisition via password to
gss_acquire_cred_from(), storing credentials in a new MEMORY ccache.

Move existing cred store tests from t_gssapi.py to t_credstore.py and
add new ones for password acquisition.

[ghudson@mit.edu: squashed commits; rewrote commit message]

ticket: 8962 (new)