Enable using envvar for username (#128)

* Update docs

* Add env variable for username

README.rst

@@ -195,11 +195,13 @@ Usage
      # connection_error_strategy [strict|skip|ignore] Choose how to handle connection errors
      # return_jira_metadata = False (return Jira issue with metadata instead of boolean result)
 
-   You can set the password field by setting the PYTEST_JIRA_PASSWORD environment variable:
+   You can set the password and username fields by setting the PYTEST_JIRA_PASSWORD and PYTEST_JIRA_USERNAME
+   environment variables:
 
     .. code:: sh
 
       export PYTEST_JIRA_PASSWORD="FOO"
+      export PYTEST_JIRA_USERNAME="BAR"
 
    Configuration options can be overridden with command line options as well.
    For all available command line options run following command.

pytest_jira.py

@@ -30,6 +30,7 @@
 IGNORE = 'ignore'
 PLUGIN_NAME = "jira_plugin"
 PASSWORD_ENV_VAR = 'PYTEST_JIRA_PASSWORD'
+USERNAME_ENV_VAR = 'PYTEST_JIRA_USERNAME'
 
 
 class JiraHooks(object):
@@ -513,7 +514,7 @@ def pytest_configure(config):
     if config.getvalue('jira') and config.getvalue('jira_url'):
         jira_connection = JiraSiteConnection(
             config.getvalue('jira_url'),
-            config.getvalue('jira_username'),
+            os.getenv(USERNAME_ENV_VAR) or config.getvalue('jira_username'),
             os.getenv(PASSWORD_ENV_VAR) or config.getvalue('jira_password'),
             config.getvalue('jira_verify'),
             config.getvalue('jira_token'),