fix(config_migrate): always remove domain old key & replace old value by new value

[scruel]

This fixed the following issues for _migratedomainconf function:

• it won't be able to remove empty old keys
• it will set the old value to new key, even the value of new key already set in the config file.

For example, if we have the following config file:

SAVED_TEST=''
SAVED_Test=''
SAVED_Test1=''
SAVED_TEST1=''
SAVED_Test2=''
SAVED_Test3='old'
SAVED_TEST4='new'
SAVED_Test5=''
SAVED_TEST5='new'
SAVED_Test6='old'
SAVED_TEST6=''
SAVED_Test7='old'
SAVED_TEST7='new'
SAVED_Test8='__ACME_BASE64__START_b2xk__ACME_BASE64__END_'
SAVED_TEST9='__ACME_BASE64__START_b2xk__ACME_BASE64__END_'
SAVED_Test10=''

And code:

_migratedeployconf Test TEST
_migratedeployconf Test1 TEST1
_migratedeployconf Test2 TEST2
_migratedeployconf Test3 TEST3
_migratedeployconf Test4 TEST4
_migratedeployconf Test5 TEST5
_migratedeployconf Test6 TEST6
_migratedeployconf Test7 TEST7
_migratedeployconf Test8 TEST8 'base64'
_migratedeployconf Test9 TEST9 'base64'
_migratedeployconf Test10 TEST10 'base64'

Without this PR patch, the result is messy and not right.
Config file after executed as the following:

SAVED_TEST=''
SAVED_Test=''
SAVED_Test1=''
SAVED_TEST1=''
SAVED_Test2=''
SAVED_TEST4='new'
SAVED_Test5=''
SAVED_TEST5='new'
SAVED_TEST6='old'
SAVED_TEST7='old'
SAVED_TEST9='__ACME_BASE64__START_b2xk__ACME_BASE64__END_'
SAVED_Test10=''
SAVED_TEST3='old'
SAVED_TEST8='__ACME_BASE64__START_b2xk__ACME_BASE64__END_'

With this PR patch, the result is neat and correct:

SAVED_TEST=''
SAVED_TEST1=''
SAVED_TEST4='new'
SAVED_TEST5='new'
SAVED_TEST6='old'
SAVED_TEST7='new'
SAVED_TEST9='__ACME_BASE64__START_b2xk__ACME_BASE64__END_'
SAVED_TEST3='old'
SAVED_TEST8='__ACME_BASE64__START_b2xk__ACME_BASE64__END_'