Default to python3 in the install script

[cheriimoya]

when running mininet/util/install.sh -3fy, i ran into the following problem:

  File "setup.py", line 17, in <module>
    import setuptools
ImportError: No module named setuptools

this is because the installation script is not setting the PYPKG variable at all python dependencies correctly.

so down the rabbit hole i went and fixed some more stuff in the installation script.

the commits with their commit messages should be speaking for themselves.

also

Installing ryu script to /usr/local/bin
Installing ryu-manager script to /usr/local/bin
ln: failed to create symbolic link '/usr/local/bin/ryu-manager': File exists

gets fixed by this.

i tested this change by running mininet/util/install.sh -3fy and passed