Use get_password method to obtain qpid password

Currently, qpid's password is stored in an attribute in the common
cookbook.  This change causes the password to be looked up via
get_password method, which is how we obtain the rabbitmq password.

Change-Id: Ia617f406041da7d03867be5b347fdbb52e1fb2e3
Implements: blueprint use-data-bag-for-qpid-password