Fix kolla_ansible python module check

The kolla-ansible script now (Train+) checks whether the kolla_ansible
python module is installed, and emits the following message if not:

ERROR: kolla_ansible has to be available in the PYTHONPATH (e.g.
installed)

It is supposed to exit 1 if this check fails, but does not.

This change fixes this by removing a subshell from the kolla-ansible
script.

Change-Id: I2c1a7398f2b8f876abcf2508874835154cb4ce57
Closes-Bug: #1854049