specify gateway support when create subnet

sometimes we don't use default gateway address in my subnet
server migrate support

user with admin role can migrate a server from one host to another ,by
scheduler
fix bug when instantiate NovaImage in NovaServer

After i boot a server from a cinder volume in icehouse . I want to get
the server instance with NovaServer Object, but it throw a exception
because openstack return server json where value of imgae is  empty
string, not null. Class NovaServer's imgae is instance of NovaImage,
instantiate failed and throws an exception. So I change the NovaImage to
Object, and modify getImgae() to satisfy.
Merge pull request #51 from octupszhang/master

add specify gateway support when create subnet,and server migrate function.  Issue #49 addressed in this merge