Timeout when performing time sync during client install

We use ntpd now to sync time before fetching a TGT during client
install.  Unfortuantely, ntpd will hang forever if it is unable to
reach the NTP server.

This patch adds the ability for commands run via ipautil.run() to
have an optional timeout.  This capability is used by the NTP sync
code that is run during ipa-client-install.

Ticket: https://fedorahosted.org/freeipa/ticket/4842
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>

ipaplatform/base/paths.py

@@ -186,6 +186,7 @@ class BasePathNamespace(object):
     SSLGET = "/usr/bin/sslget"
     SSS_SSH_AUTHORIZEDKEYS = "/usr/bin/sss_ssh_authorizedkeys"
     SSS_SSH_KNOWNHOSTSPROXY = "/usr/bin/sss_ssh_knownhostsproxy"
+    BIN_TIMEOUT = "/usr/bin/timeout"
     UPDATE_CA_TRUST = "/usr/bin/update-ca-trust"
     BIN_WGET = "/usr/bin/wget"
     ZIP = "/usr/bin/zip"