packaging: spec: add support for EL7 builds

- add support for EL7 builds
- avoid to require both java and java-1.7.0-openjdk
  since java is already provided by java-1.7.0-openjdk

Change-Id: Ia3bf3df726a5092637e2e0b8e2dd95625f7f89da
Bug-Url: https://bugzilla.redhat.com/1159293
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>