MEDIUM: ssl: Add "show ssl ca-file" CLI command

MEDIUM: ssl: Add "show ssl ca-file" CLI command

This patch adds the "show ssl ca-file [<cafile>[:index]]" CLI command.
This command can be used to display the list of all the known CA files
when no specific file name is specified, or to display the details of a
specific CA file when a name is given. If an index is given as well, the
command will only display the certificate having the specified index in
the CA file (if it exists).
The details displayed for each certificate are the same as the ones
showed when using the "show ssl cert" command on a single certificate.

This fixes a subpart of GitHub issue #1057.