volume expansion/contraction for tiered volumes

[ShyamsundarR]

Tiering is to have support for add and remove brick on tiered volumes. By add and remove brick, the rebalance process that are to be followed add/remove brick will also be in.All this will be in experimental state. Before the code for volume expansion gets in, we will push the code to have tier under the service framework and would have separated attach-tier from add-brick, detach-tier from remove brick and tier daemon from rebalance.

https://docs.google.com/document/d/1lj7f0aF5TS3N2I9inUBn-5p2ZpJSV4uptPnIN3KYKEo

https://docs.google.com/document/d/18jDyOIkJuifufqR5afIkZGXiTD77Gs22alK7J94SN1g

[amarts]

tier is removed from the build from glusterfs-6. If reopen if anyone is interested to work.