Allow setting dhcp_provider in devstack
Allow setting dhcp_provider in devstack

Provide a variable IRONIC_DHCP_PROVIDER to set CONF.dhcp.dhcp_provider.
This is needed to properly configure ironicclient functional tests,
which don't deploy neutron.

Change-Id: If601934d0c59b5dd5556336e5644832dc7564548
Partial-Bug: #1648179

openstack/ironic,NaohiroTamura/ironic,SauloAislan/ironic,SauloAislan/ironic,openstack/ironic,pshchelo/ironic,pshchelo/ironic,NaohiroTamura/ironic