Fix for groupadd execution module failures in SLES11 systems

[meaksh]

What does this PR do?

This PR fixes a bug in group execution module when we're using SLES11 SPX and also openSUSE 11.X systems.

gpasswd version for SLES11 SPX and openSUSE 11.X does not support the arguments passed by Salt when calling adduser, deluser or members functions of group execution module.

This PR uses usermod and groupmod instead of gpasswd to manage the groups when using these systems.

Previous Behavior

clisles11sp3-suma3pg:~ # salt-call group.adduser www root
[INFO    ] Executing command 'gpasswd --add root www' in directory '/root'
[ERROR   ] Command 'gpasswd --add root www' failed with return code: 6
[ERROR   ] output: gpasswd: unrecognized option '--add'
Try `gpasswd --help' or `gpasswd --usage' for more information.
local:
    False

New Behavior

clisles11sp3-suma3pg:~ # salt-call group.adduser www root
[INFO    ] Executing command 'usermod -A www root' in directory '/root'
local:
    True

Tests written?

No

Thanks!

/cc @isbm @kkaempf

[cachedout]

Any comments here @isbm ?

[isbm]

@meaksh Unit tests? 😉

[meaksh]

Added changes for unit tests. Now we cover some special cases for different OS 😄