Changes

fix downstream error: ImportError: cannot import name 'style' from 'cmd2'

cmd2 version 3.0 was released, with significant API changes. Limit the
dependency to below 3.0, as already reflected in requirements.txt.

Seeing but not changing the discrepancy in minimum version:
requirements.txt has >2.6.2 while setup.py has >= 1.5.0.

Related: SYS#7775 SYS#7777
Change-Id: I5186f242dbc1b770e3ab8cdca7f27d2a1029fff6