sheep/http: check http driver in http_init()

If users start sheep daemon like:

sheep -r host=127.0.0.1,port=8000 -c local

the sheep will startup normally, but when someone access
127.0.0.1:8000 by swift interface, it will get message of
"405 Method Not Allowed" without any explaination in sheep.log

So we use 'swift' as default http driver if users doesn't point out
in command line.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: fix compile warning

It will report warning:

node.c: In function ‘node_stat’:
node.c:278: warning: ‘last.r.peer_total_rx’ may be used uninitialized in this function
node.c:278: warning: ‘last.r.peer_total_tx’ may be used uninitialized in this function
node.c:278: warning: ‘last.r.peer_total_nr’ may be used uninitialized in this function
node.c:278: warning: ‘last.r.gway_total_rx’ may be used uninitialized in this function
node.c:278: warning: ‘last.r.gway_total_tx’ may be used uninitialized in this function
node.c:278: warning: ‘last.r.gway_total_nr’ may be used uninitialized in this function

when compiling sheepdog. The reason is we forget to initialize argument 'last'.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>