Consider lc string for bootable True/False

The xml.volumes_client check to translate the bootable
setting on a volume object only checks for Upper case strings
and bools.

It turns out that this test was introduced after a bug was introduced
to cinder.api which inadvertently changed the type used to set this
member field.  We now have a patch to fix this and put it back to "true/false"
(lc strings) but since the change there's a tempest test that verifies this.

This patch will check for string true/false either lc or uc, and will
keep the bool check in place as well (otherwise everything will fail).

This test did such a great job we should consider removing the other options
for true/false representations which would've caught the original error introduced
in Cinder (reference cinder lp# 1227837.

Fixes bug: 1227858

Change-Id: I8a4b1f4ce3a6a420e8152ea8c66a9b3fc127fe96
Merge "Consider lc string for bootable True/False"