Fix CI job errors due to protobuf dependencies

Fix CI job errors due to protobuf dependencies

Pin python protobuf package to 3.6.1 for all of the conductor.
By explicitly pinning the protobuf package in conductor's
requirement.txt file we force pip to install protobuf version
3.6.1, which is the minimum needed version for etcd3 to work.

This pinning prevents pip from installing a version of protobuf
3.20.X and above, which causes CI jobs to fail due to the
breaking changes introduced in 3.20.X protobuf.

Issue-ID: OPTFRA-1132
Signed-off-by: Lincoln Thurlow <lincoln@isi.edu>
Change-Id: If1b1be4810cb65be15e0b60d904b4027e6dce428