Remove configuration parameter Q_DHCP_EXTRA_DEFAULT_OPTS

According to the comments in stack.sh the configuration parameter
Q_DHCP_EXTRA_DEFAULT_OPTS should be removed during the Juno cycle.

Change-Id: I3b38ff57f00e93e6d9692f516291f7129584bc74
Merge "Remove configuration parameter Q_DHCP_EXTRA_DEFAULT_OPTS"
Merge commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch