The SELinux policy boolean 'virt_use_fusefs' is set to OFF by default in Red Hat Enterprise Linux. As a result, GlusterFS volumes cannot be attached via PackStack during or after installation by OpenStack.
Workaround: Set 'virt_use_fusefs=1' using setsebool, as follows:
setsebool -P virt_use_fusefs=1
After this setting, SELinux will allow GlusterFS attachment, and thus GlusterFS volumes can be utilized by OpenStack.