Remove 3.5 and xenial support (SC-711) (#1167)

Includes:
 - Update tox.ini and .travis.yml accordingly
 - Cleanup tox.ini with new tox syntax and cloud-init dependencies
 - Update documentation accordingly
 - Replace/remove xenial references where additional testing isn't required
 - Remove xenial checks in integration tests
 - Replace yield_fixture with fixture in pytest tests

Sections of code commented with lines like "Remove when Xenial is no
longer supported" still exist as they're require additional testing.

1 files changed, 2 insertions, 3 deletions

diff --git a/tests/integration_tests/modules/test_disk_setup.py b/tests/integration_tests/modules/test_disk_setup.py
index 22277331..8f9d5f40 100644
--- a/tests/integration_tests/modules/test_disk_setup.py
+++ b/tests/integration_tests/modules/test_disk_setup.py
@@ -20,7 +20,7 @@ def setup_and_mount_lxd_disk(instance: LXDInstance):
     )
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def create_disk():
     # 640k should be enough for anybody
     subp("dd if=/dev/zero of={} bs=1k count=640".format(DISK_PATH).split())
@@ -133,10 +133,9 @@ class TestPartProbeAvailability:
         assert sdb["children"][1]["name"] == "sdb2"
         assert sdb["children"][1]["mountpoint"] == "/mnt2"
 
-    # Not bionic or xenial because the LXD agent gets in the way of us
+    # Not bionic because the LXD agent gets in the way of us
     # changing the userdata
     @pytest.mark.not_bionic
-    @pytest.mark.not_xenial
     def test_disk_setup_when_mounted(
         self, create_disk, client: IntegrationInstance
     ):