Implement SDL CLI 'get namespaces' -command

Implement SDL CLI 'get namespaces' -command

Implement a new 'get namespaces' -command for 'sdlcli' -tool. With this
command user can list all the namespaces found in database. Command
syntax is:
  sdlcli get namespaces [flags]
Supported flags are -g, --group, -h and --help flags. With -g,--group
flag user can get a list of namespace per SDL cluster group address and
-h, --help flags shows the command help.

Issue-Id: RIC-113

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: If4d66497eeec94d479748c81cf0898c2fbc78364