Fixed computation of topic segments for MQTT DevFactory context

[tcalmant]

The MQTT device factory adds topic segments to the mapping context, e.g. foo/bar gives context.topic-0 set to foo.
This was failing if the the topic was starting with a /, e.g. /foo/bar.

This PR fixes the segmentation process, following the MQTT specification:

• /foo, foo and foo/ are different
• with a / prefix, context.topic-0 is always an empty string
• with a / suffix, context.topic-last is always an empty string
• foo/bar and foo//bar are equivalent, but foo/bar and foo/ /bar are different (and context.topic-1 will be a space character)