Revert "skip test_volume_boot_pattern"

This reverts commit cb6c99f31174f0cc3b2c20c9eeef721b0cabb790.

Bug 1270608 is fixed by Cinder change Ib615847f now so enable
test_volume_boot_pattern.

Related-Bug: #1270608

Change-Id: Ic67995d64bead7f318a9c054f9dd6b25ca10bd4e
Merge "Revert "skip test_volume_boot_pattern""