Add PS1 colours for centosvm

(This is a partial cherry-pick of commit 9b2bcb50260069570225ac7774030aa127b3b1fa)
Improve __git_ps1 behaviour

Use __git_ps1 to build PS1 using PROMPT_COMMAND, per the header to
git-prompt.sh.  Enable all the options!
(cherry picked from commit 54a23c9c3d84389325a077128a61f83d495598f6)
Remove unused `git_colour`

The `git_colour` variable used to set the colour of the Git info part of
PS1, but hasn't been used in ages.  Remove it.
(cherry picked from commit 2a3b51418f6721bcb936e17c145b5d0db5ff144e)
Change CentosVM prompt colours

The CentosVM prompt colours would put multiple parts in red next to each
other if the current directory wasn't part of a Git repository.  That's
not helpful.  Don't do that.
(cherry picked from commit 424b64011656fa3740211abddd71eeee8e68c60b)
Don't specify an explicit path to vim

When setting the VISUAL variable, just specify "vim" rather than giving
an explicit path.  Whichever one is in $PATH is probably the one we want
to use.  That's certainly the case on the current CentosVMs.
(cherry picked from commit 7ebcebfe9a8b59301e11fb7a69b2555e9b54a231)
Handle PS1 on Tastycake

On Tastycake, __git_ps1 doesn't work as part of PROMPT_COMMAND as
expected.  I can't immediately see what's going on, so for now, use a
regular PS1 and cope with the fact that the Git part isn't going to be
coloured in.