Update credential script to add ability to set maximum reservation time

[val500]

Description

Updates the client credentials script to allow for adding max_reservation_time. Follows similar logic to the job_priority part of the script.

Resolved issues

Resolves https://warthogs.atlassian.net/browse/CERTTF-459

Documentation

N/A

Web service API changes

N/A

Tests

Tested locally with a local deployment of Testflinger Server/MongoDB

[plars]

Looks ok, consistent with the other things at least. Needs the other part of this to land (see comments from George) and also seems to have a minor lint issue to fix

[plars]

One minor thing in several places that I just noticed. it's worth cleaning up now while we wait on the other part to land.

[plars]

I just noticed this, because the + blended in with the other "+"s on the left :)
But you don't really need that + to concatenate the strings. Any string within parentheses is going to get automatically concatenated. This is similar to you how can handle things like line length limits by adding ()s and breaking a string across lines. It's a minor thing but let's go ahead and clean it up since we're also waiting for the other thing to land anyway.