common.docker: Improve OpenSSL and CMake install introducing helper scripts

To accommodate the requirements associated with x86 and x64 images, the
command building OpenSSL and CMake became overly complex and hard to
maintain.

This commit has multiple purposes:

(1) simplify common.docker

(2) fix the building of 64-bit shared libraries against the static openssl
libraries by passing the -fPIC flag.

(3) ensure [many]linux-x86 and [many]linux-x64 images have an up-to-date
OpenSSL install. Openssl static libraries are installed in /usr

(4) simplify and speedup CMake build avoiding the second build with
explicit -DCMAKE_USE_OPENSSL:BOOL=ON. Indeed, configuring CMake on Linux
already looks for OpenSSL.

(5) speedup download of CMake source directly downloading the archive
corresponding to the revision.

(6) test CMake by:
  - running CMake.FileDownload test
  - trying to download a file served over https

.gitignore

@@ -2,3 +2,5 @@ bin
 dockcross
 */test/
 Dockerfile
+*/install-openssl.sh
+!imagefiles/install-openssl.sh