AC_AttitudeControl: ensure yaw gets a FLTD default

[andyp1per]

When people come to tune yaw D they need to have a FLTD set. There is no downside to not setting this by default

[amilcarlucas]

I have it on the tuning guide, but a default is even better

[lthall]

After the meeting I believe the only change needed for clarity is the name of the variable to include Y for yaw. AC_ATC_MULTI_RATE_RP_FILT_HZ to AC_ATC_MULTI_RATE_RPY_FILT_HZ

[peterbarker]

I've cherry-picked Leonard's fixes for the naming into this PR.

In addition to fixing the defines in AP_CustomControl he's fixed the same defines in Copter.

I've tested that there's no compiler output change vs Andy's patch, so I believe that the one objection brought up at DevCall has been addressed. I'm marking this as MergeOnCIPass.