Helm: fail fast when Openshift admin token is not configured

[kanovotn]

When running Helm tests without setting the xtf.openshift.admin.token property, the test fails with a non-descriptive NullPointerException.

Current behavior:

java.lang.NullPointerException: Cannot invoke "String.indexOf(int)" because "value" is null
...
	at cz.xtf.core.openshift.CLIUtils.executeCommand(CLIUtils.java:51)
	at cz.xtf.core.openshift.CLIUtils.executeCommand(CLIUtils.java:45)
	at cz.xtf.core.helm.HelmBinary.execute(HelmBinary.java:51)

Expected behavior:
If xtf.openshift.admin.token is not configured, XTF/Helm integration should fail fast before invoking the Helm CLI, and throw a clear, user-friendly exception.