Removes check CONF.dhcp_options_enabled from nova

* Neutron supports extra_dhcp_opts since Havana
* Hence remove boolean check dhcp_options_enabled

Closes-Bug: #1214162
DocImpact
Change-Id: I9af217a87a237e078e70332cbed1c80ad4d5db0a
Merge "Removes check CONF.dhcp_options_enabled from nova"