adding option for subdir export

[thotz]

Currently volume can be export via following command storhuag export

Enhance current option take subdir as option as well.

So option will look like
storhaug export [subdir path]
It should be optional, by default it will take root as paremeter.

For the ease of management the it is better to store subdir configuration as different file than in same export conf file. So the way of storing export conf also need to be changed.

Following is one of the suggestion exports of each volume should belong to new directory, say if we have volume vol we need to export three subdirs subdir1, subdir2,subdir3 and "/" as well. The directory hierarchy will be
/exports/vol
|---- root.conf
|---- export.subdir1.conf
|-----export.subdir2.conf
|-----export.subdir3.conf