selftest: Store fork-and-exec daemon info in a hashmap

selftest: Store fork-and-exec daemon info in a hashmap

This intermediary refactor adds a hashmap that stores the values needed
to run each samba daemon. This adds a bit more code in the short term,
but it basically means the code in 5 different places now becomes
identical, and we can extract it out to a common function.

The converting FULL_CMD from an array reference back to an array is a
bit ugly, but we can clean this up a bit once the code is all in one
place.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>