Fix a problem with 'volume list' when 'all_tenants=0'

This change checks the value of 'all_tenants' and returns a list of volumes
for all tenants when 'all_tenants' is 1 (or True). A tenant-specific volume
list is returned when 'all_tenants' is 0 (or False).

An InvalidInput exception is thrown if the 'all_tenants' value is not 0, 1,
True, or False (case insensitive).

Provided new unit tests for the get_all api which check the all_tenants, and
limits parameters.

Fixed pep8 compliance test fails with code being improperly indented, and
changed calls using str() to use six.text_type()

Change-Id: If0b8c343ee7e4e05c9aec89241458dbdf2e4734b
Closes-Bug: #1342046
Merge "Fix a problem with 'volume list' when 'all_tenants=0'"