Allow for prettytable 0.7.x as well

Relax requirements to >= 0.6, < 0.8, as 0.7.x seems to
work as well. Added testcase to ensure this in glanceclient.

Change-Id: Ib7f6d99ede09846072a6df8328b7c317a1826946
Merge "Allow for prettytable 0.7.x as well"