Add support of arm for doctor

Add support of arm for doctor

This is half of the fix to make the doctor suite run on arm.
The other half is at https://gerrit.opnfv.org/gerrit/#/c/20369/
In this fix I set an enviroment variable, to be used by
the doctor suite when ran with functest. The variable is set only
when present in config_functest.yaml, which we use as a custom file
when running functest on arm.

Change-Id: If34d3e7734a194bf9fe8b525a362e029767caa4f
Signed-off-by: cristinapauna <cristina.pauna@enea.com>