sheep: add an option for selecting the output directory of sheep.log

Previously sheep.log was saved in the Sheepdog store directory,
and used size of Sheepdog store directory included log file size.
So, administrator couldn't do capacity planning simply.

This patch adds an option for setting the output directory of sheep.log.

Usage:
	$ sheep -L dir=<log_output_dir>

Example:
	$ sheep -L dir=/logdir /datastore

[ Fixed a over-80 char style by using "?:" operator - Yuan]

Signed-off-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: merge options related to sheep.log

Previously, sheep had many log options, -l, -F , -d and -L.

This patch merge options to the option '-l'.

Usage:
	$ sheep -l dir=<log_directory>,level=<log_level_num>,format=<log_format>

Example:
	$ sheep -l dir=/logdir,level=7,format=server

Signed-off-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>