Update descriptions to clarify image contents and reduce maintenance.

[jdeathe]

Dockerfile org.deathe.description metadata
Replace:
CentOS-7 7.6.1810 x86_64 - SCL, EPEL and IUS Repositories / Supervisor / OpenSSH.
With:
OpenSSH 7.4 / Supervisor 4.0 / EPEL/IUS/SCL Repositories - CentOS-7 7.6.1810 x86_64.

README-short.txt
Replace:
CentOS-6 6.10 x86_64 / CentOS-7 7.6.1810 x86_64 - SCL/EPEL/IUS Repos / Supervisor / OpenSSH.
With:
OpenSSH / Supervisor / EPEL/IUS/SCL Repos - CentOS.

CHANGELOG.md
Replace:
## centos-7
With:
## 2 - centos-7

Replace summary contents with generic content that can be reused on all images.

Summary of release changes.