Replace qpid_host with qpid_hostname

This patch replaces `qpid_host` with `qpid_hostname` in both the
documentation and the config file sample. `qpid_host` has been shipped
as the parameter to specify qpid's hostname, although the correct one is
`qpid_hostname`.

DocImpact
Closes-bug 1242715

Change-Id: I25f2d0e505560e1762f7f059728c33c47a23439e
(cherry picked from commit 21ea5f6535dd484fa7934d07f3519fbda7e5f6ee)
Merge "Replace qpid_host with qpid_hostname" into stable/havana