Substitute environment variable in 'service' name

[IlyaFinkelshteyn]

No description provided.

[IlyaFinkelshteyn]

This will enable scenario like this:

environment:
 matrix:
   - sqlver: mssql2014
   - sqlver: mssql2016

services:
  - $(sqlver)

[photodude]

This form would also be nice to have for these kinds of multiple SQL version testing scenarios

environment:
  matrix:
  - db: mssql
    driver: sqlsrv
    db_version: sql2008r2sp2
    php: 7.2
    services: mssql2008r2sp2
  - db: mssql
    driver: sqlsrv
    db_version: sql2012sp1
    php: 7.2
    services: mssql2012sp1

[photodude]

Thank you @IlyaFinkelshteyn for filing this. I believe this will be a good improvement.