This patch is a followup to this[1] change.
The reasons for removing amp_hw_arch from octavia_post:
1. In stable/queens, we break since we don't feed ansible with any
value.
2. We don't support or ship an Amphora image that is non-x86_64.The
purpose of specifying the arch is (as stated here[1]) to hint the Nova
scheduler which compute nodes to take into account when it schedules
an Amphora instance.
Closes-Bug: #1824368
[1] Ia7be6503a40e08d0d1f7f4d89132c9e9b5bd6704
Change-Id: I36a851397a0adeadc3216656c96c58f8668fd7fc