Replace slow usage of gettext by faster one

The Python docs suggest against the current usage.
In profiles it has shown that this is very slow as for every translation
request the translation file is searched and an exception thrown as none
is found.
The change caches the translation which would be (temporarily) created by _gettext (on every call) and reuses it.
If none is found a fallback (NullTranslation) is automatically used.

easybuild/base/generaloption.py

@@ -40,13 +40,30 @@
 from functools import reduce
 from optparse import Option, OptionGroup, OptionParser, OptionValueError, Values
 from optparse import SUPPRESS_HELP as nohelp  # supported in optparse of python v2.4
-from optparse import gettext as _gettext  # this is gettext.gettext normally
 
 from easybuild.base.fancylogger import getLogger, setroot, setLogLevel, getDetailsLogLevels
 from easybuild.base.optcomplete import autocomplete, CompleterOption
 from easybuild.tools.py2vs3 import StringIO, configparser, string_type
 from easybuild.tools.utilities import mk_rst_table, nub, shell_quote
 
+try:
+    import gettext
+    eb_translation = None
+
+    def get_translation():
+        global eb_translation
+        if not eb_translation:
+            # Finding a translation is expensive, so do only once
+            domain = gettext.textdomain()
+            eb_translation = gettext.translation(domain, gettext.bindtextdomain(domain), fallback=True)
+        return eb_translation
+
+    def _gettext(message):
+        return get_translation().gettext(message)
+except ImportError:
+    def _gettext(message):
+        return message
+
 
 HELP_OUTPUT_FORMATS = ['', 'rst', 'short', 'config']