add projectKey from env if not defined in properties

[oko-x]

problems in #19 is resolved here, if the file sonar-project.properties contains projectKey variable, it won't set from Env

reason for this change is to fix backward incompatible removal of ENV variable that I was using in many CI builds which would require update on multiple projects

new code supports definition of projectKey in this order from lowest priority:

• default value $CI_PROJECT_NAME
• step env variable $SONAR_PROJECT_KEY
• projectKey in sonar-project.properties