Slight increase in testenv disk space

When building multiple overcloud control nodes the undercloud needs more
disk space for image handling. The current default of 30gb is very close
to this limit and causes spurious out of space failures. Increasing this
value to 40.

Change-Id: Ic515e176748bd4ef644188e7e30a35ed38c2c141
Merge "Slight increase in testenv disk space"