src/ostree: Add --group option to ostree config

Fetching value from a repo config using 'ostree config
get SECTIONNAME.KEYNAME' didn't work in some cases like
when having dots in Group Name entry.
As per Desktop entry file specification, Group Name
may contain all ASCII characters except for [ and ]
and control characters.
Link - https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-1.1.html

Having --group option will help user to clearly specify
Group Name and get desired result.

It also adds test for ostree config get|set and bash
completion for --group option

Fixes https://github.com/ostreedev/ostree/issues/1565

Closes: #1696
Approved by: cgwalters

bash/ostree

@@ -398,6 +398,7 @@ _ostree_config() {
     "
 
     local options_with_args="
+        --group
         --repo
     "