This commit fixes the test system hanging due to
the galera_var_notify_ssl_ipv6 test and also brings
the wsrep_notify[_ssl].sh files in line with each other
between the user template and the mtr suite.

Quotes are also added here to avoid problems if the
user specifies the value of one of the variables at the
beginning of the file containing shell-specific characters,
for example, if the password or username specified in the
PSWD and USER variables will contain the "$" character.

Also fixed an issue with automatic --ssl-verify-server-cert
option substitution when the corresponding value is set
by the user to "1" or "on".

Also fixed some tests here to avoid joining one of the nodes
to another cluster when the nodes are restarted from the mtr
side, which can lead to random failures when testing with
buildbot.