Add Sheepdog store

Sheepdog is a distributed block storage. This patch enables Sheepdog cluster
as a backend store for glance.

Test:

You can set up a simulated 3 node cluster on the local machine with
following script:

 $ sudo apt-get install liburcu-dev
 $ git clone git://github.com/collie/sheepdog.git
 $ cd sheepdog
 $ ./autogen.sh; ./configure --disable-corosync
 $ make; sudo make install
 $ for i in 0 1 2; do sheep /tmp/store$i -n -c local -z $i -p 700$i;done
 $ collie cluster format

Then change the default store in glance-api.conf as sheepdog

blueprint: add-sheepdog-support

Change-Id: I99907bbfc2e131146de9dd1a39f94a73cd2585e9
Merge "Add Sheepdog store"