NEXUS-5828: Upgrade step for security manager

Changes:
* upping security config version from 2.0.4 to 2.0.5
* added upgrade step, that sets security manager to "default"
* cleanup of Upgrade203to204
NEXUS-5828: Fixing upgrade steps

Upgrade203to204 was setting wrong model version ("current" instead of 2.0.4)
Upgrade204to205 was not setting model version, and actual change made
optional, as it seems tests never set this value, but perform content
comparison on resulting file.
NEXUS-5828: Updating resources

Attached default configuration version was
wrong, was not having proper version.

Same for some UT resources used during tests.
NEXUS-5828: Updating test resources

As they are used to compare content of resulting
config file, and failed (version mismatch)
NEXUS-5828: Added since tags, javadoc
NEXUS-5828: Fixed javadoc, wrong class linked
NEXUS-5828: Removed securityManager from sec-conf.xml

As we concluded, there is really no need to have it
exposed, nor user configurable, as we have only one
of these.
NEXUS-5828: Resource and UT update

As now the fact of missing Realm is revealed
(before, injected map was simply empty), some UTs
needed to be changed, to extend propared support
class.
NEXUS-5828: Reverted class hierarchy in case of two UTs

As here, NexusAppTestSupport cannot be used, since
UTs perform some preparations that would simply
happen "too late", when components are alreeady
looked up (and in case of Timeline, legacy upgrade
happens in init step.)
NEXUS-5828: LDAP common tests fix

ITs (basically slow UTs) of nexus ldap common
fixed
Merge pull request #97 from sonatype/nexus-5828-security-manager-upgrade

NEXUS-5828: Upgrade step for security manager