Bluetooth: shell: Split GATT commands

This splits GATT command under 'bt' to 'gatt' removing 'gatt-' prefix
from commands:

gatt - Bluetooth GATT shell commands
Options:
  -h, --help  :Show command help.
Subcommands:
  discover-characteristic  :[UUID] [start handle] [end handle]
  discover-descriptor      :[UUID] [start handle] [end handle]
  discover-include         :[UUID] [start handle] [end handle]
  discover-primary         :[UUID] [start handle] [end handle]
  discover-secondary       :[UUID] [start handle] [end handle]
  exchange-mtu             :[none]
  read                     :<handle> [offset]
  read-multiple            :<handle 1> <handle 2> ...
  subscribe                :<CCC handle> <value handle> [ind]
  write                    :<handle> <offset> <data> [length]
  write-without-response   :<handle> <data> [length] [repeat]
  write-signed             :<handle> <data> [length] [repeat]
  unsubscribe              :[none]
  metrics                  :register vendr char and measure rx
			    [value on, off]
  register                 :register pre-predefined test service
  show-db                  :[none]
  unregister               :unregister pre-predefined test service

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>