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: If95c39af79d49894503631fb8acadb4bf8cd0d45
Implements: blueprint use-data-bag-for-qpid-password