fix(driver): replace relative path by absolute path for default ssh private key.
fix(driver): replace relative path by absolute path for default ssh
private key.

Go dont expand the tilde '~' in paths, which resulted in a 'file not found'
error when using the default value of the flag 'g5k-ssh-private-key'.
Removing the tilde and using Docker machine mcnutils to get the home
directory corrected this behavior.

gfieni/docker-machine-driver-g5k