Cellular: add cellular network tests

[jarvte]

Description

• added more cellular network unit tests
• added cellular greentea tests
• fixed error in unit tests in master
• fixed CellularPower:: set_power_level(int func_level, int do_reset = 0) do_reset to '0' instead of '1' so it will not break the API.
• minor fixes to AT_CellularNetwork class which were found while doing tests

@mirelachirica @AnttiKauppila please review

Internal ref to defect: IOTCELL-917-947

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[jarvte]

continuous-integration/travis-ci/pr failure not related to this pr

[0xc0170]

continuous-integration/travis-ci/pr failure not related to this pr

Restarted. Having some travis failures recently, we reported it to Travis, waiting for their feedback

[jarvte]

@0xc0170 Please launch morph

[0xc0170]

/morph build

[0xc0170]

what is this fixing - changing the default value?

[jarvte]

So that it's not an API change we used default value but it was wrong in this pr: #7146
Here it's corrected and now it does not change the API behaviour.

[mbed-ci]

Build : SUCCESS

Build number : 2441
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7269/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2072
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7269/

[mbed-ci]

Test : SUCCESS

Build number : 2220
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7269/2220