Make MaintenanceInfo a pointer
Make MaintenanceInfo a pointer

so we don't include an empty struct when it's not configured

[#161091710]

Signed-off-by: Derik Evangelista <70505a31063ee00d819d9d243caf579a659cbb4f@pivotal.io>
Co-authored-by: Derik Evangelista <70505a31063ee00d819d9d243caf579a659cbb4f@pivotal.io>

pivotal-cf/brokerapi,pivotal-cf/brokerapi