Add config 'mountoptions'

Hi ! 

This commit add config `mountoptions` to be able to change mount options `defaults` to any value.

## Sample Usage

```ruby
config.persistent_storage.enabled      = true
config.persistent_storage.location     = "./tmp/sourcehdd.vdi"
config.persistent_storage.size         = 500
config.persistent_storage.mountname    = 'xfs'
config.persistent_storage.filesystem   = 'xfs'
config.persistent_storage.mountpoint   = '/mnt/xfs'
config.persistent_storage.mountoptions = ['defaults', 'prjquota']
```

```
[vagrant@vagrant-centos65 ~]$ cat /etc/fstab 

#
# /etc/fstab
# Created by anaconda on Thu Dec  5 14:15:50 2013
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
UUID=ecd27adf-074b-4511-8cee-4a08a4620a4f /                       ext4    defaults        0 0
UUID=6a229de0-2206-4e10-b1a2-496ea60ec30e swap                    swap    defaults        0 0
tmpfs                   /dev/shm                tmpfs   defaults        0 0
devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
sysfs                   /sys                    sysfs   defaults        0 0
proc                    /proc                   proc    defaults        0 0
/dev/vps-vg1/xfs /mnt/xfs xfs defaults,prjquota 0 0
```