[CI] fix No such property: APM_URL_BASE

[v1v]

What

When empty params then groovy exception

Stacktrace

11:03:49  GitHub has been notified of this commit’s build result
11:03:49  
11:03:49  Also:   groovy.lang.MissingPropertyException: No such property: APM_URL_BASE for class: groovy.lang.Binding
11:03:49  Also:   groovy.lang.MissingPropertyException: No such property: APM_URL_BASE for class: groovy.lang.Binding
11:03:49  Also:   groovy.lang.MissingPropertyException: No such property: APM_URL_BASE for class: groovy.lang.Binding
11:03:49  groovy.lang.MissingPropertyException: No such property: APM_URL_BASE for class: groovy.lang.Binding
11:03:49  	at groovy.lang.Binding.getVariable(Binding.java:63)
11:03:49  	at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onGetProperty(SandboxInterceptor.java:270)
11:03:49  	at org.kohsuke.groovy.sandbox.impl.Checker$7.call(Checker.java:353)
11:03:49  	at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:357)
11:03:49  	at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:333)
11:03:49  	at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:333)
11:03:49  	at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:333)
11:03:49  	at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:333)
11:03:49  	at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:333)
11:03:49  	at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.getProperty(SandboxInvoker.java:29)
11:03:49  	at com.cloudbees.groovy.cps.impl.PropertyAccessBlock.rawGet(PropertyAccessBlock.java:20)
11:03:49  	at WorkflowScript.run(WorkflowScript:68)

Tests

With some empty values

[elasticmachine]

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Build stats

• Build Cause: [Started by user Victor Martinez]

• Start Time: 2020-10-22T13:13:25.907+0000

• Duration: 51 min 8 sec

Test stats 🧪

Test	Results
Failed	0
Passed	10836
Skipped	6098
Total	16934

[v1v]

It seems the apm-url is required to be used:

[2020-10-22T12:34:14.228Z] fatal: [tester-awslinux]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'apm_url_base' is undefined\n\nThe error appears to be in '/var/lib/jenkins/workspace/Beats_beats-tester-mbp_PR-185/src/roles/common/tasks/main.yml': line 9, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Compute vars (redhat)\n  ^ here\n"}

I'll need to add the option to disable the apm-server validation otherwise we might end up with failures that are not related to the beats but the apm-server one