Also upgrading version of grpcio in system tests.
The library supports backwards compatibility, so the code
runs fine without change.

In order to actually execute the code, some rewritten imports had to be
dealt with (they were referenced by the original import name in a
function call).

Ran some `sed` commands to avoid adding extra hacks to `rewrite_imports.py`:

git grep -l ' google\.longrunning\.' | xargs sed -i 's/ google\.longrunning\./ gcloud_bigtable\._generated\./g'
git grep -l ' google\.protobuf\.empty_pb2\.' | xargs sed -i 's/ google\.protobuf\.empty_pb2\./ gcloud_bigtable\._generated\.empty_pb2\./g'

Again ran some `sed` commands to avoid adding extra hacks
to `rewrite_imports.py`:

git grep -l ' google\.bigtable\.admin\.cluster\.v1\.' | xargs sed -i 's/ google\.bigtable\.admin\.cluster\.v1\./ gcloud_bigtable\._generated\./g'

Again ran some `sed` commands to avoid adding extra hacks
to `rewrite_imports.py`:

git grep -l ' google\.bigtable\.admin\.table\.v1\.' | xargs sed -i 's/ google\.bigtable\.admin\.table\.v1\./ gcloud_bigtable\._generated\./g'

Again ran some `sed` commands to avoid adding extra hacks
to `rewrite_imports.py`:

git grep -l ' google\.bigtable\.v1\.' | xargs sed -i 's/ google\.bigtable\.v1\./ gcloud_bigtable\._generated\./g'

The alias is no longer needed since all stubs have
been converted to the new implementation.

If it doesn't exist, the test setup catches an error and then
creates the cluster.

Also fixing tll->ttl typo.