Permit configuration of DHCP when creating subnets

Allow operators to control if a subnet will have DHCP enabled by
adding support for a 'enable_dhcp' boolean, which will get passed
down to Neutron when creating the subnet.

Drive-by a spelling correction of 'tenant'.

Change-Id: I1dad5e65983839066b472ad6f634cf2ef51c715c
Merge "Permit configuration of DHCP when creating subnets"