Add retries to DataSourceGCE.py when connecting to GCE

[vteratipally]

Add retries to DataSourceGCE.py when connecting to GCE

summary: 

Add retries to DatasourceGCE when connecting to GCE. Sometimes when the trying to fetch the metadata, the cloud-init fails and the fallback datasource NoCloud is used which is not expected. So adding retries to ensure loading of the data source. 

Additional Context

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[blackboxsw]

Thanks you for this submission @vteratipally. I've confirmed that you have signed the cloud-init Contributor Agreement.

I've provided a couple of comments about how we can best add this retry logic to GCE in a way that aligns with how other datasources instrument this setting in a more flexible way.

Please also add the following:

1. your github username alphabetically in ./tools/.github-cla-signers
2. unit test coverage of these changes if you can in tests/unittests/test_datasource/test_gce.py
3. documentation update to doc/rtd/topics/datasources/gce.rst with a new section called

Configuration                                                                   
-------------  
The following configuration can be set for the datasource in system             
configuration (in `/etc/cloud/cloud.cfg` or `/etc/cloud/cloud.cfg.d/`).         
                                                                                
The settings that may be configured are:                                        
...

See either doc/rtd/topics/datasources/openstack.rst or doc/rtd/topics/datasources/ec2.rst for examples of the markdown in this section.

[blackboxsw]

Changes, docs and tests look good to me. will test this tomorrow and approve if no other concerns. Much thanks @vteratipally

[blackboxsw]

After long delay, finally got round to validating behavior on GCP. Thank you, thank you for this change and your patience.

[vteratipally]

thank you @blackboxsw