Found out the solution. This is not a single line command though

1) Find out the mpack.staging.path by running

cat /etc/ambari-server/conf/ambari.properties | grep -i mpack 

2) Go to the mpack.staging.path directory (default is : /var/lib/ambari-server/resources/mpacks )

cd /var/lib/ambari-server/resources/mpacks

3) Iterate through all the directories in the mpack staging directory excpet the "cache" directory and read the mpack.json in each directory

The mpack.json has the name of the mPack.

You can write a small script to iterate through the directories and print all the mpack names

Thanks,

Aditya