Fix bug 2412: fix identification of bootable volume

The volume identification done during the test Basic28 to identify
the root volume and the ordinary volume fails, leading to a sequence of
failed test cases.

After upgrading pip dependencies or after recent changes in the VIM, the
command line used to get a volume and check if it is bootable is
returning "True", while the if clause is comparing its value with "true".

openstack volume show ${volume_id} | grep bootable | awk '{print $4}'

This commit fixes it by converting the obtained value to lower case before
the comparison.

Change-Id: Ie6c8d11fb27fbd0fe10e256bc6688a7cd5cb5388
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>