new: default to "." instead of the full os.getcwd

This give us smaller relative paths in the output and lets us include
the default in the help generated by argparse (including the full
absolute path returned by os.getcwd would vary depending on where the
user runs 'zpm new -h'.)
deploy: standardize default in cmdline help texts

We now use %(default)s where possible to let argparse insert the
correct default in the help text.

The flags that default to an environment variable list the environment
variable instead of the actual value -- listing the default would not
be helpful when the environment variable isn't set (it would be the
empty string).