Support running an image without a context

[NatalieYuGong]

Fixes #287

This checklist is used to make sure that common guidelines for a pull request are followed.

• The PR has modified HISTORY.rst describing any customer-facing, functional changes. Note that this does not include changes only to help content. (see Modifying change log).

• I adhere to the Command Guidelines.

[djyou]

Do you want to send this PR to https://github.com/Azure/azure-cli? /cc @sajayantony

[sajayantony]

If we have covered all the scenarios and ACKd from folks on this team.

[northtyphoon]

Can you send PR to https://github.com/Azure/azure-cli?

[sajayantony]

@NatalieYuGong - could you also update the help commands on how to use -f - and run with --cmd
Also call out the impact of using /dev/null.
Do we need to add test cases for these?

[NatalieYuGong]

Can you send PR to https://github.com/Azure/azure-cli?
Sent Azure#8814

[NatalieYuGong]

@NatalieYuGong - could you also update the help commands on how to use -f - and run with --cmd
Also call out the impact of using /dev/null.
Do we need to add test cases for these?
Updated.
'Help' has the /dev/null being the default value for a contextless run/task. Will call out the impact later.
Added.