Add optional-requirements.txt

While 'optional requirements' phrase is somewhat self-contradicting,
that's what we really have: requirements of optional (pluggable) parts
of TaskFlow are optional in the sense that one can use TaskFlow
without them.

This change puts all such requirements into optional-requirements.txt to
have all them documented in one place, uses that file to list deps for
default python 2.7 test environment.

Change-Id: Ibeeb6ac68247697790cbc2a2c24d2e9f59a4479c
Merge "Add optional-requirements.txt"