Feature mussel xcreate

It's difficult to build vif params and set vif params to mussel optoin.
because building vif params is very complex.

This change is a solution to be able to set vif params using new "xcreate" command.

Example:

```
$ export MUSSEL_CUSTOM_DATA="
     --data-urlencode image_id=${image_id} \
     --data-urlencode instance_spec_name=${instance_spec_naem} \
     --data-urlencode security_groups[]=${security_groups} \
     --data-urlencode ssh_key_id=${ssh_key_id} \
     --data-urlencode hypervisor=${hypervisor} \
     --data-urlencode cpu_cores=${cpu_cores} \
     --data-urlencode memory_size=${memory_size} \
     --data-urlencode display_name= \
     --data-urlencode host_name= \
     --data-urlencode vifs[eth0][index]=0 \
     --data-urlencode vifs[eth0][network]=${network_id} \
  "
$ mussel.sh instance xcreate
```