inspiral.py : pick necessary env variables to store in the coinc dosument

@patrick.godwin brought up an idea to pick some of the env variables to store in the coinc document rather than storing everything and to avoid the unnecessary disk/memory usage. This commit implements the feature in the inspiral.py.

To test this behavior, using chunk14 C00 data set, I ran

gstlal_inspiral --singles-threshold inf --reference-psd gstlal_reference_psd/12477/H1L1V1-REFERENCE_PSD-1247740052-4164.xml.gz --track-psd  --psd-fft-length 4 --output ~/projects/gstlal_envvar_test/lloid_test_chunk14C00.xml.gz --gps-end-time 1247740152 --fir-stride 1 --channel-name=H1=GDS-CALIB_STRAIN_CLEAN --data-source frames --gps-start-time 1247740052 --time-slide-file tisi.xml --tmp-space _CONDOR_SCRATCH_DIR --svd-bank ~/projects/gstlal_envvar_test/H1-0000_SVD-1247739088-790492.xml.gz --ranking-stat-output ~/projects/gstlal_envvar_test/rankingstat_test_chunk14C00.xml.gz --veto-segments-file vetoes.xml.gz --local-frame-caching  --control-peak-time 0 --min-instruments 1 --frame-cache frame.cache --frame-segments-name datasegments --frame-segments-file segments.xml.gz --coincidence-threshold 0.005

The output lloid database is

/home/leo.tsukada/projects/gstlal_envvar_test/lloid_test_chunk14C00.xml.gz

in CIT, which looks to properly reflect the env variables.