shell: Add option to keep current environment

When doing interactive development via kas shell it is often desirable
to keep the user's customized configuration. The new --preserve-env
argument has been added to support this scenario via an opt in flag.

This flag is blocked  when not running from a TTY or via kas-container
and kas issues a warning to the user about potential unintended side
effects when invoked.

Signed-off-by: Ryan Fairfax <rfairfax@linux.microsoft.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/libcmds.py

@@ -148,6 +148,14 @@ class SetupHome(Command):
         Sets up the home directory of kas.
     """
 
+    # A list of environment variables that SETUP_HOME uses
+    # This should be kept up to date with any code in execute()
+    ENV_VARS = [
+        'GIT_CREDENTIAL_HELPER',
+        'AWS_CONFIG_FILE',
+        'AWS_SHARED_CREDENTIALS_FILE',
+    ]
+
     def __init__(self):
         super().__init__()
         self.tmpdirname = tempfile.mkdtemp()