Custom executor UI fix

[ridv]

With this patch two things are done:

1. The executor name is exposed through the getTasksWithoutConfigs API. This is necessary for the UI to distinguish between Thermos based executors which expect a certain sandbox layout, and non-thermos based executors which do not have this concept.

This adds minimal overhead (in the case of thermos constant overhead) and doesn't reveal any more information than was previously known or obtainable through the getTaskStatus.

2. Renders links to the thermos observer only if the executor used is thermos.
   Used a bold tag when non-thermos tasks are rendered for the Mesos agent address in order to maintain UI consistency.

[StephanErb]

LGTM. Please squash your commits before merging :)

[ridv]

Thanks @StephanErb. The friendly neighborhood @jcohen made sure can only use the "Squash and merge" strategy so no more worries about wonky history 🙂