Fix lower-constraints job

oslotest 1.5.1 is restricted to pbr <1.0 which conflict
with other deps lower constarint for pbr.

Error:
The conflict is caused by:
    oslo-i18n 3.15.3 depends on pbr!=2.1.0 and >=2.0.0
    oslo-serialization 2.21.1 depends on pbr!=2.1.0 and >=2.0.0
    oslo-utils 4.5.0 depends on pbr!=2.1.0 and >=2.0.0
    oslotest 1.5.1 depends on pbr!=0.7, <1.0 and >=0.6

https: //zuul.opendev.org/t/openstack/build/cb2bce3b4e004601958b764dfd0737b8
Change-Id: I513262b85a57ce9f81a63bd778fdd7f7be27e485