samba-tool: copy user_keytrust to computer keytrust

samba-tool: copy user_keytrust to computer keytrust

This is exactly a copy of user/keytrust.py to computer_keytrust.py
with a title-case-preserving `s/user/computer/`.

It works. The Computer model differs from the User model in that it
appends a '$' to the end of account names if it senses the lack,
otherwise these commands are using the same code paths.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>