Under your VPN Authentication you have the default role for each service.
(Aruba7030) *[mynode] # show aaa authentication vpn
VPN Authentication Profile List
-------------------------------
Name References Profile Status
---- ---------- --------------
default 0
default-cap 0 Predefined
default-hp-switch 0 Predefined
default-iap 0 Predefined
default-rap 0 Predefined
Then you would assign the IP Pool in question to the User Role:
(Aruba7030) *[mynode] (config) #user-role default-rap
(Aruba7030) ^*[mynode] (config-submode)#pool l2tp
STRING Pool name