ci: increase storage in GH runners and run integration tests in different machines

[DnPlas]

This PR includes the following changes:

• ci: increase storage of GH runners for integration testing

Increasing the storage of the GH runners will avoid potential issues when running integration
tests that deploy SeldonDeployments that require a significant amount of them.

Part of #185

• ci: run integration tests in different machines to avoid deploying seldon twice

Building and deploying seldon twice in a row may lead to errors while executing test cases from two different
test files. This change ensures each individual test file runs on its own environment.

Part of #185

Expected CI failures

• Integration tests may fail during the charm build stage because we are missing changes introduced by ci, build: update CI, libraries, python dependencies, and format #184. To make sure that other CI runs without failures, this PR should be merged first.

[i-chvets]

Approved. Pending CI.