To test bootable flag in a cinder volume

This commit adds to cinder client a bootable capability
supported flags are  True or False.
And a testcase for changing the bootable flag from true to false.

Update bootable volume is not supported on icehouse
Added new flag under config.py - to enbale this feature by request
Change-Id: Ia23cab169c46d631444399ab1edd93c43cfb7ef5
4 files changed