NSX: fix API payloads for dhcp/metadata setup

NSX 4.2 GA has tweaked the way dhcp and metadata
ports are configured. This patch takes care of
that.

Closes-bug: #1312822

Change-Id: I4e8496bf721c0f35df4228ec9c6f8066275e8c7a
Removed signing_dir from neutron.conf

The default value from keystone middleware ($TMPDIR) is good enough,
so stick to it.

Closes-Bug: 1313780
Change-Id: Ideb393f9010f4f1706392ccdfdd5fa7307bda070
Merge "Removed signing_dir from neutron.conf"
Merge "NSX: fix API payloads for dhcp/metadata setup"