Support 'infinite' dhcp_lease_duration

Process a dhcp_lease_duration value of -1 as 'infinite'
when setting the dnsmasq dhcp-range values to support
cases where it is undesirable for instance dhcp leases
to expire.

DocImpact

Closes-Bug: #1315430
Change-Id: I1cc3cfebfec355014e6d5b5cbd656a1300c43c0b
Revert "fix openvswitch requirement check"

This reverts commit be340d1bab015c47650687f97393c6c9015fb537.

The fix did not fix anything.
See comments at
https://review.openstack.org/#/c/95592/

Change-Id: I2926acf4b0a7020a7ad3237e6d03d5b78bfcb6c4
Merge "Support 'infinite' dhcp_lease_duration"
Merge "Revert "fix openvswitch requirement check""