set JAVA_OPTS_KC_HEAP in order to not override all JVM opts in Keycloak container

[jcarranzan]

Summary

According this issue comment : #2106 (comment)
it's better set JAVA_OPTS_KC_HEAP in order to not override all JVM opts in Keycloak container

Please select the relevant options.

• Bug fix (non-breaking change which fixes an issue)
• Dependency update
• Refactoring
• Backport
• New scenario (non-breaking change which adds functionality)
• This change requires a documentation update
• This change requires execution against OCP (use run tests phrase in comment)

Checklist:

• Methods and classes used in PR scenarios are meaningful
• Commits are well encapsulated and follow the best practices

[michalvavrik]

LGTM