fix: Change juju 3 support strategy

[jneo8]

• Now all juju 3 will use 3/stable snap release
• Update functional test to run all the supported versions by zaza
• Drop bionic support

[dashmage]

Since this testing strategy is a bit more different than what we're used to with other charms, it would be nice to include some documentation around this -- how we're testing the various juju versions, libjuju, zaza details, about env vars like TEST_JUJU_CHANNEL, TEST_JUJU etc.

[jneo8]

Since this testing strategy is a bit more different than what we're used to with other charms, it would be nice to include some documentation around this -- how we're testing the various juju versions, libjuju, zaza details, about env vars like TEST_JUJU_CHANNEL, TEST_JUJU etc.

I feel it's should be document in zaza, not here. Right?

[dashmage]

Since this testing strategy is a bit more different than what we're used to with other charms, it would be nice to include some documentation around this -- how we're testing the various juju versions, libjuju, zaza details, about env vars like TEST_JUJU_CHANNEL, TEST_JUJU etc.

I feel it's should be document in zaza, not here. Right?

I was thinking more along the lines of how we are using zaza + other details specifically for these tests. But if it seems trivial, then we can ignore my comment. The changes LGTM :)

[jneo8]

Cover by #52