OET robustness improvements in support of SS-82

Initial ideas for robustness improvements:

• Improve handling of missing / unresponsive SKUID service (fall back to internal IDs? reject requests entirely? Would also need modifications to REST CLI for this to be user-friendly)
• Guards to assert that target device state is as expected before attempting a command (does not eliminate race conditions - state at cmd exec time may not be same as at read time. Responsibility better placed with server rather than client. Adds complexity that could be addressed by better design).
• Terminate commands if state transitions to a state outside the set allowed for happy path + sad path
• Introduce timeouts to catch the most egregious problems with unresponsive devices and long-running commands (could be tricky for some transitions. how long should a command take? ideally, progress events should be emitted by target device. to come with long-running commands feature?)
• Minimise event subscriptions (aiming for one long-lived subscription per device attribute per script rather than one transient subscription per request - multiple subscriptions suspected to have problems with Tango)
• Use SKALLOP test fixtures for setup + teardown (would give better, standardised handling of faults and errors during test setup)
• Better exception handling and messages, reporting the likely cause of a problem where understood
• Increase robustness of OET CLI to malformed user input
• Improved OET CLI reporting of deployment errors 
• Better test runner logging that retrieve stack trace via 'oet describe' when a test fails so this can be reported alongside the test that failed.

Selected for PI12:

• Better test runner logging that retrieve stack trace via 'oet describe' when a test fails so this can be reported alongside the test that failed.
• Minimise event subscriptions (aiming for one long-lived subscription per device attribute per script rather than one transient subscription per request - multiple subscriptions suspected to have problems with Tango)
• Terminate commands if state transitions to a state outside the set allowed for happy path + sad path