This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2020-05-06 17:35:56,751 synthtool > Executing /tmpfs/src/github/synthtool/working_repo/synth.py.
On branch autosynth
nothing to commit, working tree clean
.coveragerc
.flake8
.github/CONTRIBUTING.md
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.github/PULL_REQUEST_TEMPLATE.md
.github/release-please.yml
.gitignore
.kokoro/build.sh
.kokoro/continuous/common.cfg
.kokoro/continuous/continuous.cfg
.kokoro/docs/common.cfg
.kokoro/docs/docs.cfg
.kokoro/presubmit/common.cfg
.kokoro/presubmit/presubmit.cfg
.kokoro/publish-docs.sh
.kokoro/release.sh
.kokoro/release/common.cfg
.kokoro/release/release.cfg
.kokoro/trampoline.sh
CODE_OF_CONDUCT.md
CONTRIBUTING.rst
LICENSE
MANIFEST.in
docs/_static/custom.css
docs/_templates/layout.html
docs/conf.py.j2
docs/multiprocessing.rst
noxfile.py.j2
renovate.json
setup.cfg
Running session blacken
Creating virtual environment (virtualenv) using python3.6 in .nox/blacken
pip install black==19.3b0
black docs google tests noxfile.py setup.py
reformatted /tmpfs/src/github/synthtool/working_repo/docs/conf.py
reformatted /tmpfs/src/github/synthtool/working_repo/noxfile.py
All done! ✨ 🍰 ✨
2 files reformatted, 15 files left unchanged.
Session blacken was successful.
2020-05-06 17:36:00,462 synthtool > Wrote metadata to synth.metadata.

```
</details>

This PR adds support for variable create and update methods.

The create method has been implemented in the Variable class instead of Config (which would have given symmetry with get_variable) for a couple of reasons:

it allows a more natural workflow of calling config.variable(), setting the desired attribute, then saving
it updates the created variable in place via _set_properties like other variable methods
This also adds support for the text attribute, which can now be used in alternative to the previously supported value. The create and update methods enforce mutual exclusivity of the two attributes.

Fixes #1

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>