Automatically install argparse if missing.

argparse is available from python2.7, but a lot of python2.7 compilations does
not provide it.

As users expect package to work by default, in case if python used to install
package does not contain argparse, add it to install_requires without requiring
any intervention, because default invocation shall work.

setup.py

@@ -10,6 +10,13 @@ long_description = open("README.txt").read() + "\n" + \
 for f in sorted(glob.glob(os.path.join('slapos', 'README.*.txt'))):
   long_description += '\n' + open(f).read() + '\n'
 
+# Provide a way to install additional requirements
+additional_install_requires = []
+try:
+  import argparse
+except ImportError:
+  additional_install_requires.append('argparse')
+
 setup(name=name,
       version=version,
       description="SlapOS toolbox.",
@@ -34,7 +41,7 @@ setup(name=name,
         'slapos.core', # as it provides library for slap
         'xml_marshaller', # needed to dump information
         'GitPython', #needed for git manipulation into slaprunner
-      ],
+      ] + additional_install_requires,
       extras_require = {
         'lampconfigure':  ["mysql-python"], #needed for MySQL Database access
         'zodbpack': ['ZODB3'], # needed to play with ZODB