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: I7afe9a0a069c6527ced4df7f094c31c1ca4acec4
Implements: blueprint use-data-bag-for-qpid-password
Merge "Use get_password method to obtain qpid password"