* feat(helm): add $HELM_KEY_PASSPHRASE environment variable for signing helm charts

If $HELM_KEY_PASSPHRASE is set then helm package sign command will not prompt the
user to enter the passphrase for the private key
Signed-off-by: Anumita Shenoy <ansheno@microsoft.com>

* docs(helm):  added documentation for HELM_KEY_PASSPHRASE

Added description for HELM_KEY_PASSPHRASE
Signed-off-by: Anumita Shenoy <ansheno@microsoft.com>