System tests with custom agent version

[teresaromero]

Since #3054 system tests are able to override the agent version. In order to test a package using different agent versions, we should implement these tests.

Some options to implement this test are:

• save the agent version flag from the stack command into the profile and re-use it when running tests
• create a config entry to be able to set this param into the system tests configuration

Related thread #3054 (comment)