Fix nova-bm-dnsmasq.service

[/usr/lib/systemd/system/nova-bm-dnsmasq.service:9] Unknown lvalue
'-ExecStartPre' in section 'Service'

Adding a - to ExecStartPre causes systemd to ignore
the exit code but the "-" was in the wrong place.

Change-Id: I2a62f48b307f09aa23603d97066e6a1c9c76135f