added `title_image: default` everywhere

mission/cluster_backbone.md

 ---
 title: HIFIS Backbone Cluster
+title_image: default
 layout: default
 excerpt:
     Overview of HIFIS Backbone Cluster.

mission/cluster_cloud.md

 ---
 title: HIFIS Cloud Cluster
+title_image: default
 layout: default
 excerpt:
     Overview of HIFIS Cloud Cluster.

mission/cluster_software.md

 ---
 title: HIFIS Software Cluster
+title_image: default
 layout: default
 excerpt:
     Overview of HIFIS Software Cluster.

mission/incubator.md

 ---
 title: Helmholtz Incubator
+title_image: default
 layout: default
 excerpt:
     Detailed description of the HIFIS governance structures.

mission/index.md

 ---
 title: HIFIS Mission
+title_image: default
 layout: default
 excerpt:
     Overview of HIFIS Motivation, Structure and Governance.

mission/members_FedBoard.md

 ---
 title: HIFIS Federation Board
+title_image: default
 layout: default
 excerpt:
     Detailed description of the HIFIS governance structures.

mission/members_SAB.md

 ---
 title: HIFIS SAB
+title_image: default
 layout: default
 excerpt:
     Detailed description of the HIFIS governance structures.