add bash completion for easyconfigs from local dir but not robot search path

[SebastianAchilles]

This PR adds bash completion for easyconfigs only from local dir but not robot search path. This option might be useful if you are working on a slow shared filesystem and searching the entire robot search path is too slow.

This bash completion can be enabled with:

source $EBROOTEASYBUILD/bin/minimal_bash_completion.bash;
source $EBROOTEASYBUILD/bin/optcomplete.bash;
source $EBROOTEASYBUILD/bin/eb_bash_completion_local.bash;
complete -F _eb eb

[bartoldeman]

lgtm