When grenade.sh is invoked by devstack-gate, PYTHON3_VERSION
is not set so grenadrc defaults it to 3.5, which breaks grenade
on nodes where python 3.5 does not exist, such as ubuntu bionic.
One solution is to set PYTHON3_VERSION in devstack-gate [1].
Alternatively, this change uses a bit of code borrowed from
stackrc in devstack to determine the version of python
within grenade if it was not set by the caller.
Closes-bug: #1812208
[1] https://review.openstack.org/#/c/607379/
Change-Id: I0349de2026c49279ba7f262d5e86d37018d66326
d5dc033a2e