Restrict prettytable to >=0.6,<0.8.
Restrict prettytable to >=0.6,<0.8.

Match the openstack/requirements version of prettytable in pip-requires
to avoid version conflicts.

Change-Id: I78f3c6ef9a9a8d35939eb100827e429c12a0a60d

alexpilotti/python-keystoneclient,metacloud/python-keystoneclient,sdpp/python-keystoneclient,ntt-sic/python-keystoneclient,darren-wang/ksc,magic0704/python-keystoneclient,citrix-openstack-build/python-keystoneclient,ging/python-keystoneclient,citrix-openstack-build/python-keystoneclient,magic0704/python-keystoneclient,metacloud/python-keystoneclient,citrix-openstack-build/python-keystoneclient,klmitch/python-keystoneclient,ntt-sic/python-keystoneclient,jamielennox/python-keystoneclient,Mercador/python-keystoneclient,ging/python-keystoneclient,sdpp/python-keystoneclient,alexpilotti/python-keystoneclient,ntt-sic/python-keystoneclient,darren-wang/ksc,jamielennox/python-keystoneclient,klmitch/python-keystoneclient,Mercador/python-keystoneclient,jamielennox/python-keystoneclient