code coverage for FileStore::_detect_fs when running on ext4

unit tests are added in test/filestore/store_test.cc for the
FileStore::_detect_fs method, when using ext4. It tests the following
situations:

 * without user_xattr, ext4 fails
 * mounted with user_xattr, ext4 fails if filestore_xattr_use_omap is false
 * mounted with user_xattr, ext4 succeeds if filestore_xattr_use_omap is true

The tests are grouped in an ext4 dedicated class

  TEST(EXT4StoreTest, _detect_fs)

The qa/workunits/filestore/filestore.sh script is added to prepare the
environment required for the unit tests ( create an image file,
formats it with ext4 etc.). It runs ceph_test_filestore with a sudo
to allow it to mount(2) and umount(2) the ext4 file system. It is
called with

  ceph_test_filestore --gtest_filter=EXT4StoreTest.*

to only run the ext4 dependent tests.

The filestore.sh script is meant to be used as part of teuthology
in order to increase the code coverage for src/os/FileStore.cc. It is
self tested and can be checked from the source directory with

  CEPH_TEST_FILESTORE=../../../src/ceph_test_filestore filestore.sh TEST

http://tracker.ceph.com/issues/4321 refs #4321

Signed-off-by: Loic Dachary <loic@dachary.org>