Skip tests in integration.shell.test_master
Skip tests in integration.shell.test_master

Related to https://github.com/saltstack/salt-jenkins/issues/378

This is likely causing all of the flakiness we're seeing on test runs
when the master stops responding.

These tests should be in their own, isolated run, rather than with
the rest of the test suite.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt