Using a single point of reference for DEFAULT_LINUX_PROMPT

Using a single point of reference for DEFAULT_LINUX_PROMPT

the idea is to have all prompts defaults come from a single place
in Variables.py.  This is only the first patch for this work.  Two
goals here:

1.  remove all local definitions of the default prompt and point
those suites/libs to use the DEFAULT_LINUX_PROMPT in Variables.py

2.  convert all default args (e.g. ${prompt}=${linux_prompt} to use
this new ${DEFAULT_LINUX_PROMPT}

future patches can be more specific and start referencing variables
not yet introduced for things like ${MININETVM0_PROMPT},
${MININETVM1_PROMPT}, ${ODLVM2_PROMPT}, etc.

Change-Id: Ic87dfb930b25b37b1e02c50750a7a2f6732b24bf
Signed-off-by: Jamo Luhrsen <jluhrsen@redhat.com>