Snapshot/Restore: make it possible to delete snapshots with missing metadata file

Fixes #7980
[TEST] Fix config directory for external nodes in bwc tests

The logging configuration was expected in the path.home folder which is set to
target/JX
when running the bwc tests from the console.
Therefore the logger could not be initialized with error message:

[INFO] Failed to configure logging...
org.elasticsearch.ElasticsearchException: Failed to load logging configuration
        at org.elasticsearch.common.logging.log4j.LogConfigurator.resolveConfig(LogConfigurator.java:117)
        at org.elasticsearch.common.logging.log4j.LogConfigurator.configure(LogConfigurator.java:81)
        at org.elasticsearch.bootstrap.Bootstrap.setupLogging(Bootstrap.java:96)
        at org.elasticsearch.bootstrap.Bootstrap.main(Bootstrap.java:180)
        at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:32)
Caused by: java.nio.file.NoSuchFileException: /home/britta/es/target/J0/config
        at sun.nio.fs.UnixException.translateToIOException(UnixException.java:86)
        at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
        at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
        at sun.nio.fs.UnixFileAttributeViews$Basic.readAttributes(UnixFileAttributeViews.java:55)
        at sun.nio.fs.UnixFileSystemProvider.readAttributes(UnixFileSystemProvider.java:144)
        at sun.nio.fs.LinuxFileSystemProvider.readAttributes(LinuxFileSystemProvider.java:97)
        at java.nio.file.Files.readAttributes(Files.java:1686)
        at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:109)
        at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:69)
        at java.nio.file.Files.walkFileTree(Files.java:2602)
        at org.elasticsearch.common.logging.log4j.LogConfigurator.resolveConfig(LogConfigurator.java:107)
        ... 4 more
log4j:WARN No appenders could be found for logger (node).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.

Setting the config directory fixes this.

Logs from external nodes are still not printed properly. They are inserted to the log
whenever the stdout is printed ([WARNING] JVM J0: stdout was not empty...)

closes #7964
[TEST] fix CurrentTestFailedMarker to reset its state after each test

The currently used method `testRunStarted` is only called before any tests have been run, we need to reset that state before each test, that's why we need to use `testStarted`.
plugins: disable support for config dir

When removing and installing again the plugin all configuration files will be removed in `config/pluginname` dir.
This is bad as users may have set and added specific configuration files.

During an install, if we detect already existing files in `config/pluginname` directory, we simply copy the new file to the same dir but we append a `.new` at the end.

Related to  #5064.

(cherry picked from commit 5da028f)
(cherry picked from commit 4cb1f95)