Dev/aamstutz/add instances

[amstuta]

Description

Add resource and datasource for instance(s)

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Test A: make testacc TESTARGS="-run TestAccCloudProjectInstance_basic"
• Test B: make testacc TESTARGS="-run TestAccDataSourceCloudProjectInstances_basic"
• Test C: make testacc TESTARGS="-run TestAccDataSourceCloudProjectInstance_basic"

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or issues
• I have added acceptance tests that prove my fix is effective or that my feature works
• New and existing acceptance tests pass locally with my changes
• I ran succesfully go mod vendor if I added or modify go.mod file

[rbeuque74]

Add a check of flavor availability in the tests to make sure they are not failing if the stock of this flavor is zero