Make remote command shell act as a login shell

This commit adds a parameter to the run_command() method call. The
value differs from the pssh's default '$SHELL -c' by adding '-l'
to make the shell act as a login shell.

This results in loading .bash_profile or .profile, depending on
which shell it is and other circumstances.

The reason to load a profile file is to set up environment, for
example PATH.

This is especially handy in environments where Python virtual envs
are used. There, to use one, the user has to activate the env by
sourcing a file.

medusa/orchestration.py

@@ -65,7 +65,7 @@ def pssh_run(self, hosts, command, hosts_variables=None, ssh_client=None):
                                 cert_file=cert_file)
             logging.debug('Batch #{i}: Running "{command}" on nodes {hosts} parallelism of {pool_size}'
                           .format(i=i, command=command, hosts=parallel_hosts, pool_size=len(parallel_hosts)))
-            output = client.run_command(command, host_args=hosts_variables,
+            output = client.run_command(command, host_args=hosts_variables, shell='$SHELL -cl',
                                         sudo=medusa.utils.evaluate_boolean(self.config.cassandra.use_sudo))
             client.join(output)