Improving help text for taskflow executor opts.
Improving help text for taskflow executor opts.

Adding improved help texts for the config options engine_mode and
max_workers

Partial-Bug: #1570946

Change-Id: I2c14301ac3ed881dc80c0e9bff3a092bc1cf0bc5

stevelle/glance,stevelle/glance,rajalokan/glance,openstack/glance,openstack/glance,rajalokan/glance,openstack/glance