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

[IamPete1]

Its a bit upsetting that its using RATE_RP (roll pitch) but filt T is the same....

In fact filt_E is useing AC_ATC_MULTI_RATE_YAW_FILT_HZ and filt_T_hz is using AC_ATC_MULTI_RATE_RP_FILT_HZ its all a bit odd....

[lthall]

Yeh, I can see how this is a little confusing or unclear. What would you suggest?

The end result is we want roll, pitch and yaw filter T and D to be the same (20.0)
roll and pitch filter E to be 0.0
yaw filter E to be 2.0

There is an opportunity here to make this clear for the future.

[andyp1per]

LOL, I originally did a bigger change that changed the names but figured this was better for 4.5 :-P

[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.