rest-api: Add get plain text crush map

Signed-off-by: Gregory Meno <gregory.meno@inktank.com>
cthulhu: moves text crush map into the osd_map sync object

Signed-off-by: Gregory Meno <gregory.meno@inktank.com>
split this up

Signed-off-by: Gregory Meno <gregory.meno@inktank.com>