Telnet: add parameter to allow stripping the prompt

The "Read Until Prompt" and "Execute Command" keywords return the output
and including the prompt. This patch adds a new parameter "strip_prompt" to
these keywords which if set to true strips the prompt from the output.

Fixes #1874.
should this be added to "execute keyword", too? And what do you mean with "at the end"? At the end of the description or paragraph, or should this an own paragraph?