Automate ISO updates for deployment tests

Now Fuel ISO update process on https://ci.fuel-infra.org is going on in half-manual mode.

Needed steps are:
- manually download needed ISO version
- change symlink on freshly downloaded Fuel ISO file
- run 'devops.create-env' job

Regular ISO update procedure takes up to 5hr

Mentioned manual steps were caused by public type of https://ci.fuel-infra.org/ Jenkins instance, where we run gate jobs
and private type of https://product-ci.infra.mirantis.net/ Jenkins instance where we're building product version of Fuel ISO

Proposed change:
- use community ISO version for deployment tests (aka gate jobs)
- put 'devops.create-env' job script in each gate job and get rid of additional 'devops.create-env' job
- improve existing gating jobs by adding an ISO download functionality inside
- adjust 'devops.env' job outcome parameters for using it in gate jobs

As a result, we'll move all preparation and ISO downloading routine directly in a gating jobs, which will check venv parameters from 'devos.env' on each run, as well as download new community ISO if mentioned in 'devops.env' job version of the ISO is not exists on Jenkins slave.

Using of community ISO version will save us around 3 hrs since community version of ISO located in a same network.
Proposed change in a jobs structure will help us to do all needed steps automatically directly in a gate jobs.
Update process will be managed only by running 'devops.env' Jenkins job with needed parameters.