This patch enables the flavor extra spec 'required:[traits]'. The
admin can specifiy a set of traits that a flavor required.
To enable this, the placement 1.17 is required, it added trait
support in the `GET /allocation_candidates` API. So bump the
minimal placement version requirement to 1.17 and update the
check in cmd `nova-status`.
Implement blueprint request-traits-in-nova
Change-Id: Ia51ace951e9a0402f873ce9751a8cd3c0283db08