fix expansion of the $hosts variable

if value of $hosts contains spaces, which can be defined by
br -m "host1 host2", bash will expand it as is. So
"if [ -z $hosts ]" is expanded to "if [ -z host1 host2 ]", this is
wrong. This commit add quotes to the $hosts variable, so it is expanded
correctly.

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>