handle repeating mysqld options containing equals

Let load_mysqld_options handle repeating options such as:
"--plugin-load=blackhole=ha_blackhole.so
--plugin-load=federated=ha_federated.so" without crashing by storing all
the mysql_options we get in a dictionary with lists as values.

Change-Id: I634532e13bab80d476e4a1e8558b23da33656f60
Closes-Bug: #1348652
Merge "handle repeating mysqld options containing equals"