Make requests_kerberos dependency optional

To avoid deployment issues on a non kerberized environment,
this change makes the `requests_kerberos` dependency optional
and requires manual explicit installation as:

   pip install yarn-api-client[kerberos]
or
   pip install requests_kerberos

Fixes #23
Closes #24

Author: lresende

README.rst

@@ -49,6 +49,18 @@ From source code
    python setup.py install
    popd
 
+
+Enabling support for Kerberos/SPNEGO Security
+
+To avoid deployment issues on a non Kerberized environment, the `requests_kerberos`
+dependency is optional and needs to be explicit installed in order to enable access
+to YARN console protected by Kerberos/SPNEGO.
+
+::
+
+   pip install requests_kerberos
+
+
 -----
 Usage
 -----

setup.py

@@ -28,8 +28,12 @@ def find_version(*file_paths):
 
     install_requires = [
         'requests>=2.7,<3.0',
-        'requests-kerberos',
     ],
+
+    extras_require = {
+        'kerberos': ['requests-kerberos'],
+    },
+
     entry_points = {
         'console_scripts': [
             'yarn_client = yarn_api_client.main:main',

yarn_api_client/base.py

@@ -5,7 +5,6 @@
 import logging
 import requests
 
-from requests_kerberos import HTTPKerberosAuth
 from .errors import APIError, ConfigurationError
 
 
@@ -34,6 +33,7 @@ def request(self, api_path, **query_args):
 
         response = None
         if self.kerberos_enabled:
+            from requests_kerberos import HTTPKerberosAuth
             response = requests.get(api_endpoint, params, auth=HTTPKerberosAuth())
         else:
             response = requests.get(api_endpoint, params)
@@ -53,6 +53,7 @@ def update(self, api_path, data):
 
         response = None
         if self.kerberos_enabled:
+            from requests_kerberos import HTTPKerberosAuth
             response = requests.put(api_endpoint, data=data, auth=HTTPKerberosAuth())
         else:
             response = requests.put(api_endpoint, data=data)