Fix description of ListSubnet

The help texts for both "neutron net-list" and "neutron subnet-list"
give the same text "List networks that belong to a given tenant."

Fix it so that subnet-list output the proper "subnet"

Change-Id: I0877b51ad10ac0e04b24effa7cbd1e412098e363
Partial-Bug: #1244618
Merge "Fix description of ListSubnet"