dog: use new environment variables for specifying IP address and port number

New environment variables are used for specifying IP address and port
number:
- SHEEPDOG_DOG_ADDR for IP address
- SHEEPDOG_DOG_PORT for port number

It is useful when sheep is launched with -b option.

If both of the variables and -a or -p option are specified, command
line option is prioritized.

Related issue:
https://bugs.launchpad.net/sheepdog-project/+bug/1322633

Cc: Valerio Pachera <sirio81@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>