openstack: neutron: convert keys to strings

engine sends integers instead of strings for configuration, make sure
these are converted to strings.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=989915
Change-Id: I8dc4d250f9d143d5b56af74a7df680970b4cf107
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>