[VCDA-789] Refactor CSE Installation - template creation (#148)

Created general use functions in utils.py. Functions in
config.py are specific to either CSE installation or config
file processing.

Split up template creation phase into independent functions.
create_template() handles '--update' option, and decides when
to create vapp, customize vapp, capture vapp, and delete vapp.

create_temp_vapp() handles the temp vapp creation and
customization phase of CSE installation. This function calls
create_vapp_from_config() and customize_vm().

create_vapp_from_config() initializes the vapp specifically for
CSE installation, using the initialization script.

customize_vm() customizes the vapp's vm using the customization
script.

capture_vapp_to_template() captures the vapp as a vapp template
to vcd catalog.

Simplified control flow and exception handling.

Fixed --update option to now properly delete previous CSE entities
(template, ova files, vapps)

Moved hard-coded values to constant module-level values.

Added descriptive console output in appropriate areas, and
created docstrings for each function.

Tested CSE install extensively. '--update' works properly,
resulting vapps and VMs can be ssh'd into, and all customization
finishes as expected.

container_service_extension/broker.py

@@ -31,6 +31,7 @@
 from container_service_extension.cluster import TYPE_MASTER
 from container_service_extension.cluster import TYPE_NFS
 from container_service_extension.cluster import TYPE_NODE
+from container_service_extension.utils import SYSTEM_ORG_NAME
 
 from container_service_extension.exceptions import ClusterOperationError
 from container_service_extension.exceptions import ClusterInitializationError
@@ -115,8 +116,10 @@ def _connect_sysadmin(self):
             verify_ssl_certs=self.verify,
             log_headers=True,
             log_bodies=True)
-        self.client_sysadmin.set_credentials(
-            BasicLoginCredentials(self.username, 'System', self.password))
+        credentials = BasicLoginCredentials(self.username,
+                                            SYSTEM_ORG_NAME,
+                                            self.password)
+        self.client_sysadmin.set_credentials(credentials)
 
     def _connect_tenant(self, headers):
         token = headers.get('x-vcloud-authorization')