charms: Introduce an integration testing suite with `scenario`

[gabrielmougard]

Using the https://juju.is/docs/sdk/scenario testing framework, we will introduce the testing of some use cases to assess the resilience of the charm against chaotic conditions:

• Three nodes random cluster initialization
• Four nodes random cluster initialization
• Three node initialized + one-unit scale-up
• Four node initialized + one-unit scale-down
• Three node initialized + multiple-units scale-up (ranging from 2 to N. Could we have N=47 ? We'd then reach a total cluster size of 50, which is the upper theoretical limit)
• More than five node cluster + multiple-units scale-down

This should be a very good start.