We have seen issues with dangling threads in Armada. This is likely due to
a bug [0] in the version of gRPC that we were pinned to.
This patchset:
- moves us to the latest versions of the gRPC python libraries which add
a new `channel.close()` method to cleanup channels.
- implements the python context manager api in the tiller handler
- uses the context manager api to explicitly scope tiller channel creation
and cleanup to each Armada API and CLI call.
This also fixes a couples issues with error handling introduced in [1].
[0]: https://github.com/grpc/grpc/issues/14338
[1]: https://review.openstack.org/#/c/610384
Change-Id: I2577a20fc76c397aa33157dc12a0e1d36f49733e
7af22df7dc